Diff-informed analysis: fix empty PR handling #3948

Job	Run time
Diagnostic export (macos-latest, linked)	1m 16s
Diagnostic export (windows-latest, linked)	4m 44s
Diagnostic export (ubuntu-latest, linked)	1m 1s
Diagnostic export (macos-latest, nightly-latest)	1m 8s
Diagnostic export (ubuntu-latest, nightly-latest)	1m 0s
Diagnostic export (windows-latest, nightly-latest)	3m 10s
	12m 19s

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