Hide empty container blocks from the course blocks api when called by the mobile app

All code in this PR should be removed after REVE-52 is merged and mobile
traffic from older app versions falls to < 5% of the mobile traffic
to the course_blocks api

lms/djangoapps/course_api/blocks/api.py

@@ -4,7 +4,9 @@ API function for retrieving course blocks data
 
 import lms.djangoapps.course_blocks.api as course_blocks_api
 from lms.djangoapps.course_blocks.transformers.hidden_content import HiddenContentTransformer
+from lms.djangoapps.course_blocks.transformers.hide_empty import HideEmptyTransformer
 from openedx.core.djangoapps.content.block_structure.transformers import BlockStructureTransformers
+from openedx.core.lib.mobile_utils import is_request_from_mobile_app
 
 from .serializers import BlockDictSerializer, BlockSerializer
 from .transformers.blocks_api import BlocksAPITransformer
@@ -60,10 +62,18 @@ def get_blocks(
 
     if user is not None:
         transformers += course_blocks_api.get_course_block_access_transformers(user)
-        transformers += [MilestonesAndSpecialExamsTransformer(
-            include_special_exams=include_special_exams,
-            include_gated_sections=include_gated_sections)]
-        transformers += [HiddenContentTransformer()]
+        transformers += [
+            MilestonesAndSpecialExamsTransformer(
+                include_special_exams=include_special_exams,
+                include_gated_sections=include_gated_sections
+            ),
+            HiddenContentTransformer()
+        ]
+
+    # TODO: Remove this after REVE-52 lands and old-mobile-app traffic falls to < 5% of mobile traffic
+    if is_request_from_mobile_app(request):
+        transformers += [HideEmptyTransformer()]
+
     transformers += [
         BlocksAPITransformer(
             block_counts,