Update serializers to remove empty choices, provide default values for fields, and better indicate the field type when possible for OPTIONS requests and browsable API docs.

awx/main/models/projects.py

@@ -203,6 +203,7 @@ class Project(UnifiedJobTemplate, ProjectOptions):
     )
     scm_update_cache_timeout = models.PositiveIntegerField(
         default=0,
+        blank=True,
     )
 
     @classmethod