Refactor get_base_url to avoid a redundant Site.find_for_request call

wagtail/api/v2/utils.py

@@ -11,8 +11,15 @@ class BadRequestError(Exception):
 
 
 def get_base_url(request=None):
-    site = Site.find_for_request(request) if request else None
-    base_url = getattr(settings, 'WAGTAILAPI_BASE_URL', site.root_url if site else None)
+    base_url = None
+
+    try:
+        base_url = getattr(settings, 'WAGTAILAPI_BASE_URL')
+    except AttributeError:
+        if request:
+            site = Site.find_for_request(request)
+            if site:
+                base_url = site.root_url
 
     if base_url:
         # We only want the scheme and netloc