[DBAL-211] Fix bug with elements in default schema not filtered for really being in the default search path.

Allow NULL value when converting a blob to PHP value.

Fix for nasty bug that happens when there are 2 or more schemas with tables that have the same name. Since all the AbstractPlatform classes get only the table name using ALL_* schema tables is dangerous because the owner is never supplied.

The other possible solution is to use the ALL_* tables but in each of the methods to fetch the owner before actualy querying them.