fix: use .get on column in mssql destination for cases where the yaml…

… does not contain the nullable property, like other sql destinations (#1380)

dlt/destinations/impl/mssql/mssql.py

@@ -181,7 +181,7 @@ def _get_column_def_sql(self, c: TColumnSchema, table_format: TTableFormat = Non
             if c.get(h, False) is True
         )
         column_name = self.capabilities.escape_identifier(c["name"])
-        return f"{column_name} {db_type} {hints_str} {self._gen_not_null(c['nullable'])}"
+        return f"{column_name} {db_type} {hints_str} {self._gen_not_null(c.get('nullable', True))}"
 
     def _create_replace_followup_jobs(
         self, table_chain: Sequence[TTableSchema]