Avoid a warning about uninitialized values

The warning is "Use of uninitialized value in join or string at
.../ensembl-test/modules/Bio/EnsEMBL/Test/DumpDatabase.pm line 222."
and happens when a table has been remove (by the patch) between the
previous and the current schema versions
Merge pull request #5 from muffato/master

Avoid a warning about uninitialized values