[BUGFIX] swiftmaileradapter should ignore empty headers

Further processing empty header-values with
functions like parseAddress() can lead to
unexpected problems.

Change-Id: Ida436f384a6a6a86d7fbd119083a9037157f48df
Fixes: #42106
Releases: 4.5, 4.7, 6.0, 6.1
Reviewed-on: https://review.typo3.org/19687
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn