indexing: Fallback to default_indexing_url in case we have no indexing_url stored for the user

Not checking for indexing_url value leads to initializing
TDB backend and this puts us in a situation where we have
MySQL backends as default, but we nevetheless use TDB for indexing
openchangedb/indexing: Handle indexing url validation in openchangedb layer

Indexing URLs must not:
- be NULL
- be empty

Also add a test case for this
openchangedb/indexing: change error returned
Merge pull request #93 from Zentyal/jgarcia/regresion-indexing-backend

Regresion indexing backend using tdb