Restore special handling of isset and empty as it was in v4. Fixes #1… #553

Job	Run time
Tests (ubuntu-latest, 7.2, default)	1m 16s
Tests (ubuntu-latest, 7.3, default)	1m 16s
Tests (ubuntu-latest, 7.4, default)	1m 23s
Tests (ubuntu-latest, 8.0, default)	1m 22s
Tests (ubuntu-latest, 8.1, default)	1m 16s
Tests (ubuntu-latest, 8.2, default)	1m 21s
Tests (ubuntu-latest, 8.3, default)	1m 24s
Tests (ubuntu-latest, 8.4, default)	1m 16s
Tests (windows-latest, 7.2, default)	1m 45s
Tests (windows-latest, 7.3, default)	1m 45s
Tests (windows-latest, 7.4, default)	1m 46s
Tests (windows-latest, 8.0, default)	1m 44s
Tests (windows-latest, 8.1, default)	1m 42s
Tests (windows-latest, 8.2, default)	1m 53s
Tests (windows-latest, 8.3, default)	1m 42s
Tests (windows-latest, 8.4, default)	1m 46s
Tests (ubuntu-latest, 8.0, jit)	1m 29s
Tests (ubuntu-latest, 8.1, jit)	1m 16s
Tests (ubuntu-latest, 8.2, jit)	1m 17s
Tests (ubuntu-latest, 8.3, jit)	1m 27s
Tests (ubuntu-latest, 8.4, jit)	1m 16s
	31m 22s

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