Added critical fixes for ASTSchemaSource class.

Modified c-tor of ASTSchemaSource class to not check text parameter with NotNull precondition.
Added NotNull preconditions check into apply methods.

Change-Id: I34965c7cd4225340d589aeebf8d22365c4d373e2
Signed-off-by: Lukas Sedlak <lsedlak@cisco.com>
Merge "Added critical fixes for ASTSchemaSource class."