grep: don't emit the -T alignment tab on empty lines

[sylvestre]

With -T, grep pads the prefix with a tab so line content lands on a tab stop. GNU omits that tab when the line has no content: an empty line prints just its prefix (a whitespace-only line still gets the tab). uu_grep always wrote the tab, so empty matched lines gained a spurious trailing tab. Gate the tab on non-empty content. Fixes the GNU testsuite 'initial-tab' test.

[github-actions]

GNU grep testsuite comparison:

Test results comparison:
  Current:   TOTAL: 128 / PASSED: 71 / FAILED: 36 / SKIPPED: 21
  Reference: TOTAL: 128 / PASSED: 70 / FAILED: 37 / SKIPPED: 21

Changes from main branch:
  TOTAL: +0
  PASSED: +1
  FAILED: -1

Test improvements (1):
  + initial-tab

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.90%. Comparing base (98e6bb6) to head (a7b1532).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #12      +/-   ##
==========================================
+ Coverage   95.82%   95.90%   +0.07%     
==========================================
  Files           6        6              
  Lines        1414     1417       +3     
  Branches      140      141       +1     
==========================================
+ Hits         1355     1359       +4     
  Misses         58       58              
+ Partials        1        0       -1     

Flag	Coverage Δ
macOS_latest	96.72% <100.00%> (+0.07%)	⬆️
ubuntu_latest	96.72% <100.00%> (+0.07%)	⬆️
windows_latest	0.00% <0.00%> (ø)

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[github-actions]

GNU grep testsuite comparison:

Test results comparison:
  Current:   TOTAL: 128 / PASSED: 71 / FAILED: 36 / SKIPPED: 21
  Reference: TOTAL: 128 / PASSED: 70 / FAILED: 37 / SKIPPED: 21

Changes from main branch:
  TOTAL: +0
  PASSED: +1
  FAILED: -1

Test improvements (1):
  + initial-tab