feat: unset default BelongsToMany relations when not loaded

Author: laylatichy

packages/database/src/Mappers/SelectModelMapper.php

@@ -50,11 +50,11 @@ public function map(mixed $from, mixed $to): array
             foreach ($model->getRelations() as $relation) {
                 // When a nullable BelongsTo relation wasn't loaded, we need to make sure to unset it if it has a default value.
                 // If we wouldn't do this, the default value would overwrite the "unloaded" value on the next time saving the model
-                if (! $relation instanceof BelongsTo) {
-                    continue;
-                }
-
-                if (! $relation->property->isNullable()) {
+                if ($relation instanceof BelongsTo) {
+                    if (! $relation->property->isNullable()) {
+                        continue;
+                    }
+                } elseif (! $relation instanceof BelongsToMany) {
                     continue;
                 }