Allow empty `<extensions>` element in XML configuration #5052

Job	Run time
Coding Guidelines	33s
Type Checker	27s
Unit Tests (ubuntu-latest, 7.3, false)	18s
Unit Tests (ubuntu-latest, 7.4, false)	16s
Unit Tests (ubuntu-latest, 8.0, false)	11s
Unit Tests (ubuntu-latest, 8.1, false)	22s
Unit Tests (ubuntu-latest, 8.2, false)	12s
Unit Tests (windows-latest, 7.3, false)	41s
Unit Tests (windows-latest, 7.4, false)	39s
Unit Tests (windows-latest, 8.0, false)	55s
Unit Tests (windows-latest, 8.1, false)	3m 16s
Unit Tests (windows-latest, 8.2, false)	34s
Unit Tests (ubuntu-latest, 8.3, true)	45s
Unit Tests (windows-latest, 8.3, true)	32s
End-to-End Tests (ubuntu-latest, 7.3, false)	25s
End-to-End Tests (ubuntu-latest, 7.4, false)	27s
End-to-End Tests (ubuntu-latest, 8.0, false)	26s
End-to-End Tests (ubuntu-latest, 8.1, false)	26s
End-to-End Tests (ubuntu-latest, 8.2, false)	26s
End-to-End Tests (windows-latest, 7.3, false)	38s
End-to-End Tests (windows-latest, 7.4, false)	43s
End-to-End Tests (windows-latest, 8.0, false)	1m 1s
End-to-End Tests (windows-latest, 8.1, false)	54s
End-to-End Tests (windows-latest, 8.2, false)	37s
End-to-End Tests (ubuntu-latest, 8.3, true)	1m 5s
End-to-End Tests (windows-latest, 8.3, true)	1m 0s
Code Coverage	1m 4s
Build PHAR	56s
Test PHAR (7.3, pcov)	17s
Test PHAR (7.3, xdebug)	36s
Test PHAR (7.4, pcov)	19s
Test PHAR (7.4, xdebug)	33s
Test PHAR (8.0, pcov)	17s
Test PHAR (8.0, xdebug)	34s
Test PHAR (8.1, pcov)	25s
Test PHAR (8.1, xdebug)	30s
Test PHAR (8.2, pcov)	22s
Test PHAR (8.2, xdebug)	32s
Test PHAR (8.3, pcov)	38s
Test PHAR (8.3, xdebug)	1m 3s
	25m 55s

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