check for nil (#5816) (#4123)

Signed-off-by: Modular Magician <[email protected]>

Author: modular-magician

.changelog/5816.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+sql: fixed crash when one of `settings.database_flags` is nil.
+```

google-beta/resource_sql_database_instance.go

@@ -1067,6 +1067,9 @@ func expandAuthorizedNetworks(configured []interface{}) []*sqladmin.AclEntry {
 func expandDatabaseFlags(configured []interface{}) []*sqladmin.DatabaseFlags {
 	databaseFlags := make([]*sqladmin.DatabaseFlags, 0, len(configured))
 	for _, _flag := range configured {
+		if _flag == nil {
+			continue
+		}
 		_entry := _flag.(map[string]interface{})
 
 		databaseFlags = append(databaseFlags, &sqladmin.DatabaseFlags{