EditTableDialog: Make it possible again to delete default value

Commit c29702a1b97e6fcc30651973591b7b565746a90d fixed the handling of
default values and added a check to enforce quote characters when they
are needed. This check wasn't prepare for empty default values (i.e. no
default value) and would add quote characters in this case, too. This is
fixed by this commit.

See issue #133.