Swap equals check around to prevent NPE

[demaniak]

Swapped equals check on strings around to prevent NPE and let the code handle the missing header condition as intended.

[demaniak]

This should prevent the NPE mentioned in #287

It will of course not magically make the missing header appear :)

[coder-hugo]

I don't like this type of comparing strings. As this changes the meaning of the "if-sentence". The current code says: I want to know if the value of the variable eventHeader is (equals) Merge Request Hook or Push Hook. Your code says: I want to know if Merge Request Hook or Push Hook is the value of the variable eventHeader.
Jenkins has already the apache commons-lang as a dependency so we should use the StringUtils for a null-safe and syntactical correct equals check.

[coder-hugo]

Can you just squash your commits to a single one then I'll merge this PR back.

[demaniak]

@coder-hugo oke-doke, I will try to get to that during today.

[demaniak]

huh, @coder-hugo find myself unsure how to safely squash the commits (never done before, and I quickly settle this before going home)

I'm just going to close this PR, and re-submit with single commit.