Ignore commit prefixes with an empty pattern

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.

Author: stefanhaller

pkg/gui/controllers/helpers/working_tree_helper.go

@@ -155,6 +155,9 @@ func (self *WorkingTreeHelper) HandleCommitPress() error {
 		commitPrefixConfigs := self.commitPrefixConfigsForRepo()
 		for _, commitPrefixConfig := range commitPrefixConfigs {
 			prefixPattern := commitPrefixConfig.Pattern
+			if prefixPattern == "" {
+				continue
+			}
 			prefixReplace := commitPrefixConfig.Replace
 			branchName := self.refHelper.GetCheckedOutRef().Name
 			rgx, err := regexp.Compile(prefixPattern)