Fix NullPointerException when visitListItem encounters a non-paragraph first child.

`visitListItem` in `MarkdownPositions` previously used a pattern-matching `switch` on `listItem.getFirstChild()`. If a Markdown `ListItem` starts with a non-paragraph child (such as when an empty or unfinished bullet item `/// -` has `null` as its first child), the `switch` statement throws a `NullPointerException` because it does not explicitly handle `null`.

This change replaces the `switch` with `if (listItem.getFirstChild() instanceof Paragraph paragraph)` to safely return `false` on `null` or non-`Paragraph` children without throwing an exception. It also updates `LIST_ITEM_START_PATTERN` to allow list markers without trailing whitespace at the end of the input (`$`).

Tested:
Added `markdownEmptyListItem` in `JavadocFormattingTest.java` and ran `blaze test //third_party/java_src/google_java_format/...`.
PiperOrigin-RevId: 945788396

Author: kluever

core/src/main/java/com/google/googlejavaformat/java/javadoc/MarkdownPositions.java

@@ -129,17 +129,15 @@ private boolean visitListItem(ListItem listItem) {
       verify(matcher.lookingAt());
       ListItemOpenTag openToken = new ListItemOpenTag(matcher.group(1));
       addSpan(listItem, openToken, LIST_ITEM_CLOSE_TOKEN);
-      return switch (listItem.getFirstChild()) {
-        case Paragraph paragraph -> {
-          // A ListItem typically contains a Paragraph, but we don't want to visit that Paragraph
-          // because that would lead us to introduce a line break after the list introduction
-          // (the `-` or whatever). So we visit the children and siblings of the Paragraph instead.
-          visitNodeList(paragraph.getFirstChild());
-          visitNodeList(paragraph.getNext());
-          yield true;
-        }
-        default -> false;
-      };
+      if (listItem.getFirstChild() instanceof Paragraph paragraph) {
+        // A ListItem typically contains a Paragraph, but we don't want to visit that Paragraph
+        // because that would lead us to introduce a line break after the list introduction
+        // (the `-` or whatever). So we visit the children and siblings of the Paragraph instead.
+        visitNodeList(paragraph.getFirstChild());
+        visitNodeList(paragraph.getNext());
+        return true;
+      }
+      return false;
     }
 
     private void visitHeading(Heading heading) {
@@ -258,5 +256,5 @@ public String toString() {
   // The leading \s here works around what appears to be a CommonMark bug. We shouldn't ever see
   // space at the purported start of a list item?
   private static final Pattern LIST_ITEM_START_PATTERN =
-      Pattern.compile("(?:\\s*)(([-+*]|[0-9]+[.)])\\s)");
+      Pattern.compile("(?:\\s*)(([-+*]|[0-9]+[.)])(?:\\s|$))");
 }

core/src/test/java/com/google/googlejavaformat/java/JavadocFormattingTest.java

@@ -31,6 +31,21 @@ public final class JavadocFormattingTest {
 
   private final Formatter formatter = new Formatter();
 
+  /**
+   * Tests that the formatter formats the given input string to the given expected string. Also
+   * tests that the formatter is idempotent when formatting an already formatted string.
+   */
+  private void doFormatTest(String input, String expected) {
+    try {
+      String actual = formatter.formatSource(input);
+      assertThat(actual).isEqualTo(expected);
+      String reformatted = formatter.formatSource(actual);
+      assertWithMessage("When checking idempotency").that(reformatted).isEqualTo(actual);
+    } catch (FormatterException e) {
+      throw new AssertionError(e);
+    }
+  }
+
   @Test
   public void notJavadoc() {
     String input =
@@ -1713,6 +1728,21 @@ class Test {}
     doFormatTest(input, expected);
   }
 
+  @Test
+  public void markdownEmptyListItem() {
+    assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
+    String input =
+"""
+/// A list with an empty item:
+/// - `foo`: enabled by default
+/// - `bar`: disabled by default
+/// -
+class Test {}
+""";
+    String expected = input;
+    doFormatTest(input, expected);
+  }
+
   @Test
   public void markdownFencedCodeBlocks() {
     assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();
@@ -2040,17 +2070,6 @@ class Test {}
     doFormatTest(input, expected);
   }
 
-  private void doFormatTest(String input, String expected) {
-    try {
-      String actual = formatter.formatSource(input);
-      assertThat(actual).isEqualTo(expected);
-      String reformatted = formatter.formatSource(actual);
-      assertWithMessage("When checking idempotency").that(reformatted).isEqualTo(actual);
-    } catch (FormatterException e) {
-      throw new AssertionError(e);
-    }
-  }
-
   @Test
   public void markdownBlankLinesAroundSnippetAndNoMangling() {
     assume().that(MARKDOWN_JAVADOC_SUPPORTED).isTrue();