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.

[DBAL-204] Sane refactoring of the previous idea. Removed a bunch of code again and implemented proper schema namespacing. Also got the wording right (hopefully) and fixed the algorithms.