Make primary field target adapter return early.

When there is no primary field, do not go through all fields.
First compare the field name, then the permission: comparing two strings is cheaper.
If the field name matches, stop iterating over other fields: you have already found the field.

news/1851.bugfix

@@ -0,0 +1,2 @@
+Make primary field target adapter return early.
+[maurits]

src/plone/restapi/serializer/dxcontent.py

@@ -220,23 +220,24 @@ def __init__(self, context, request):
 
     def __call__(self):
         primary_field_name = self.get_primary_field_name()
+        if not primary_field_name:
+            return
         for schema in iterSchemata(self.context):
             read_permissions = mergedTaggedValueDict(schema, READ_PERMISSIONS_KEY)
 
             for name, field in getFields(schema).items():
-                if not self.check_permission(read_permissions.get(name), self.context):
-                    continue
-
                 if name != primary_field_name:
                     continue
 
+                if not self.check_permission(read_permissions.get(name), self.context):
+                    return
+
                 target_adapter = queryMultiAdapter(
                     (field, self.context, self.request), IPrimaryFieldTarget
                 )
-                if target_adapter:
-                    target = target_adapter()
-                    if target:
-                        return target
+                if not target_adapter:
+                    return
+                return target_adapter()
 
     def get_primary_field_name(self):
         fieldname = None