Casts all text fields to string in case of null

Prevents sql errors on writing to database
2 jobs for 193-check-ext_emconf-validity-before-uploading-and-creating-an-invalid-version-record in 1 minute and 12 seconds (queued for 3 seconds)
Status Name Job ID Coverage
  Build
passed build #3678

00:00:23

 
  Layout
passed layout #3679

00:00:48