Select no tests if modified-only returns nothing

Previously it selected all tests. Now it respects that if nothing is
modified then nothing is selected.

Fix: #2449

tests/discover/data/plans.fmf

@@ -169,6 +169,14 @@ execute:
  modified-only: true
  modified-ref: 8329db0^
 
+ /empty-modified:
+ discover:
+ how: fmf
+ path: ../../..
+ ref: 8329db0
+ modified-only: true
+ modified-ref: 8329db0
+
  /distgit:
  summary: Extract distgit sources
  discover:

tests/discover/modified.sh

@@ -8,21 +8,23 @@ rlJournalStart
  rlPhaseEnd
 
  rlPhaseStartTest 'Command-line'
- rlRun 'tmt run -rdv discover --how fmf --ref 8329db0 \
+ rlRun -s 'tmt run -rdv discover --how fmf --ref 8329db0 \
  --modified-only --modified-ref 8329db0^ \
- plan -n features/core finish 2>&1 >/dev/null | tee output'
- rlAssertGrep 'summary: 1 test selected' output
- rlAssertGrep '/tests/core/adjust' output
+ plan -n features/core finish >/dev/null'
+ rlAssertGrep 'summary: 1 test selected' "$rlRun_LOG"
+ rlAssertGrep '/tests/core/adjust' "$rlRun_LOG"
  rlPhaseEnd
 
  rlPhaseStartTest 'Plan'
- rlRun 'env -C data tmt run -rdv discover \
- plan -n fmf/modified finish 2>&1 >/dev/null | tee output'
- rlAssertGrep 'summary: 1 test selected' output
- rlAssertGrep '/tests/core/adjust' output
+ rlRun -s 'env -C data tmt run -rdv discover \
+ plan -n fmf/modified finish >/dev/null'
+ rlAssertGrep 'summary: 1 test selected' "$rlRun_LOG"
+ rlAssertGrep '/tests/core/adjust' "$rlRun_LOG"
  rlPhaseEnd
 
- rlPhaseStartCleanup
- rlRun 'rm -f output' 0 'Remove tmp file'
+ rlPhaseStartTest 'No changes'
+ rlRun -s 'env -C data tmt run -rdv discover \
+ plan -n fmf/empty-modified >/dev/null'
+ rlAssertGrep 'summary: 0 tests selected' "$rlRun_LOG"
  rlPhaseEnd
 rlJournalEnd

tmt/steps/discover/fmf.py

@@ -569,6 +569,10 @@ def assert_git_url(plan_name: Optional[str] = None) -> None:
  if output.stdout:
  directories = [os.path.dirname(name) for name in output.stdout.split('\n')]
  modified = {f"^/{re.escape(name)}" for name in directories if name}
+ if not modified:
+ # Nothing was modified, do not select anything
+ self._tests: list['tmt.Test'] = []
+ return
  self.debug(f"Limit to modified test dirs: {modified}", level=3)
  names.extend(modified)
  else: