rdbms: in Database::selectSQLText, do not treat $conds = "0" as no condition

rdbms: in Database::selectSQLText, do not treat $conds = "0" as no condition

This fixes an issue that arises because empty( "0" ) is true in PHP.

The new behavior rejects any conditions that are not strings or arrays,
and lets $conds = "0" be passed to the databases as WHERE 0.

Some databases may reject this as invalid syntax, which is the expected
behavior here, instead of silently ignoring the 0, causing no condition to
be applied to the query.

Bug: T188314
Change-Id: I5bc4d7f41221a886c85e54d9da67c4c095a7d9ce