The tests in GitSCM.java for the project local settings in getGitConfigEmailToUse() and getGitConfigNameToUse() do not take an empty string into account. It seems that the default setting for a new build configuration is to have empty strings for these so that means that the global settings are ignored.
There was a fix for
JENKINS-9702 in revision 89e610b7 but this problem looks to have been present before then also.
The fix is simple just use fixEmptyAndTrim on project local values before testing whether to fallback on the global setting. Pull request coming soon.