Fix commitPrefix setting with empty pattern

[stefanhaller]

Before we changed the commitPrefix config to a list in #4261, we had this bug where the defaults section in Config.md would erroneously list the default for commitPrefix as

  git:
    commitPrefix:
      pattern: ""
      replace: ""

This was not correct, commitPrefix was a pointer, and the default for that was nil, which is not the same.

Now, some people copied and pasted the entire defaults section into their config files, setting the commitPrefix to an empty pattern (which is not the same as not setting it at all). And this caused the branch name to be filled in to the subject field for every commit; see for example #3632.

New users copying the defaults section into their config file in the current version no longer have this problem because now that commitPrefix is a list, it is no longer included in the defaults section. However, the migration that we added in #4261 would happily carry over those empty strings into a list entry, so people migrating from an older version still have the broken config in their config files.

To work around the issue, ignore commit prefix settings whose pattern is an empty string. I can't imagine a valid reason why people would actually want to set the pattern to an empty string, so I assume this only comes from the broken defaults problem described above.

[codacy-production]

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Coverage variation	Diff coverage
Report missing for c735a5e1	✅ 50.00%

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	Coverable lines	Covered lines	Coverage
Common ancestor commit (c735a5e)	Report Missing	Report Missing	Report Missing
Head commit (05a18ed)	53203	46084	86.62%

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	Coverable lines	Covered lines	Diff coverage
Pull request (#4381)	2	1	50.00%

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