(CAT-2164) Correct `yaml_syntax_validator` to error on empty file #875

Job	Run time
spec (ruby 2.7) / spec	49s
spec (ruby 3.2) / spec	50s
acceptance (ruby 2.7 \| windows-2019) / acceptance	27m 37s
acceptance (ruby 2.7 \| ubuntu-latest) / acceptance	12m 47s
acceptance (ruby 3.2 \| ubuntu-latest) / acceptance	12m 48s
acceptance (ruby 3.2 \| windows-2019) / acceptance	33m 47s
	1h 28m 38s

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