Add usage units fallback for OCP on GCP storage endpoint

[esebesto]

Jira Ticket

COST-4392

Description

This PR attempts to fix missing usage units in OCP on GCP storage endpoint when we have no OCP on GCP storage data. The missing units are apparent in UI so it would be better to fix it.

Testing

1. hit ocp on gcp storage endpoint without having any ocp on gcp storage data (make sure that the db schema is created, e.g., by creating ocp source):
   http://localhost:8000/api/cost-management/v1/reports/openshift/infrastructures/gcp/storage/
2. check metadata - you will see that usage units are missing

"total": {
            "usage": {
                "value": 0,
                "units": 0
            }

4. Checkout Branch
5. hit ocp on gcp storage endpoint again - but this time add a limit to invalidate the cache:
   http://localhost:8000/api/cost-management/v1/reports/openshift/infrastructures/gcp/storage/?limit=102
6. check metadata - you will see that usage units are now filled

"total": {
            "usage": {
                "value": 0,
                "units": "GB-Mo"
            }

[codecov]

Codecov Report

Merging #4975 (f754f06) into main (9a88d07) will not change coverage.
The diff coverage is n/a.

Additional details and impacted files

@@          Coverage Diff          @@
##            main   #4975   +/-   ##
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  Coverage   94.1%   94.1%           
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  Files        375     375           
  Lines      31170   31170           
  Branches    3704    3704           
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  Hits       29327   29327           
  Misses      1176    1176           
  Partials     667     667           

[esebesto]

/retest

[bacciotti]

LGTM

[esebesto]

/retest

[esebesto]

/retest