Align string member empty needle semantics

[He-Pin]

Motivation

std.member accepts either an array or a string as its first argument. For strings, the official Jsonnet stdlib defines membership through std.findSubstr(x, arr), and std.findSubstr("", "abc") returns an empty array. That means an empty string should not be considered a string member.

sjsonnet's native implementation delegated directly to JVM String.contains, which treats the empty string as contained in every string. This made std.member("abc", "") and std.member("", "") return true, even though the official stdlib-derived semantics return false.

Modification

• Require the string needle in std.member to be non-empty before calling String.contains.
• Add regression coverage for empty string needles in string membership.
• Add array membership coverage to show that std.member([""], "") remains true.

Result

Implementation	std.member("abc", "")	std.member("", "")	std.member([""], "")	std.findSubstr("", "abc")
cpp-jsonnet	false	false	true	[]
go-jsonnet	true	true	true	[]
jrsonnet	false	false	true	[]
sjsonnet before this PR	true	true	true	[]
sjsonnet after this PR	false	false	true	[]

sjsonnet now matches cpp-jsonnet and jrsonnet for string membership with an empty needle, while preserving normal substring membership and array element membership.

References

• Jsonnet stdlib docs for std.member: https://jsonnet.org/ref/stdlib.html#std-member
• Jsonnet stdlib docs for std.findSubstr: https://jsonnet.org/ref/stdlib.html#std-findSubstr
• Official Jsonnet stdlib member definition: https://github.com/google/jsonnet/blob/master/stdlib/std.jsonnet
• go-jsonnet native member implementation: https://github.com/google/go-jsonnet/blob/master/builtins.go