dockerfile/linter: check for nil linter in linter functions #4996
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During parsing, the linter can be nil and linter rules won't succeed at running due to a nil dereference. Instead of checking for nil everywhere, this modifies the linter to automatically disable itself if the linter is nil.
Fixes a nil dereference panic happening when parsing
ENV
andLABEL
commands without a linter introduced by6cfa459.
An alternative to #4984.