Guard against null iterator in toMap to prevent SIGSEGV

[ericraio]

Motivation:

cass_iterator_from_map can return NULL when the column value is non-null but not a valid map type (e.g., corrupted data or type mismatch). The existing null check on the column value (cass_value_is_null) does not cover this case. When cass_iterator_next is called with a NULL iterator, it dereferences a null pointer causing a SIGSEGV (Signal 11, exit code 139) crash.

This was observed where the server repeatedly crashed when decoding map columns from Cassandra rows with unexpected data states.

Modifications:

Changed let iterator = cass_iterator_from_map(self.rawPointer) to guard let iterator = cass_iterator_from_map(self.rawPointer) else { return nil } in the toMap method. This matches the defensive pattern already used in Data.swift for cass_iterator_from_collection.

Result:

When cass_iterator_from_map returns NULL, toMap now safely returns nil instead of crashing with a null pointer dereference. Callers that access map properties will get nil and can fall back to alternative decoding paths.