No empty SQL as argument when call executeSQL()

https://github.com/cloudant/sync-android/issues/7

1. Add condition check about empty SQL statement

2. Convert to schemas to String[]. That way we do
not need the extra parsing step to get each sql,
and there fore avoid emptry SQL statement endup
in the parsing results.
Merge pull request #8 from cloudant/7-empty-sql

Do not allow empty SQL as input for SQLDatabase.execSQL()