(CAT-2164) Correct `yaml_syntax_validator` to error on empty file #871

Job	Run time
spec (ruby 2.7) / spec	49s
spec (ruby 3.2) / spec	43s
acceptance (ruby 2.7 \| ubuntu-latest) / acceptance	12m 42s
acceptance (ruby 3.2 \| windows-2019) / acceptance	32m 35s
acceptance (ruby 3.2 \| ubuntu-latest) / acceptance	12m 46s
acceptance (ruby 2.7 \| windows-2019) / acceptance	28m 5s
	1h 27m 40s

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