[FLINK-3732] [Core] Potential null deference in ExecutionConfig#equals()

[skyahead]

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[StephanEwen]

Code looks good, but it triggers a checkstyle violation: indentation includes a space.
Please have a look at the Travis CI logs...

[StephanEwen]

Looks good. Failed test seems to be a Scala compiler issue, not related to this pull request.

Merging this...