feat(ddm): Return None when no data is present

[iambriccardo]

This PR changes how the timeseries are filled when no data is there. Now, instead of using the identity value of the totals, they are returning null.

Closes: https://github.com/getsentry/sentry/issues/63667

[codecov[bot]]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (beb644f) 81.39% compared to head (0cc9708) 81.39%. Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #64995      +/-   ##
==========================================
- Coverage   81.39%   81.39%   -0.01%     
==========================================
  Files        5249     5249              
  Lines      232053   232056       +3     
  Branches    45563    45563              
==========================================
+ Hits       188881   188883       +2     
  Misses      37286    37286              
- Partials     5886     5887       +1     

Files	Coverage Δ
...try/sentry_metrics/querying/data/transformation.py	98.07% <100.00%> (ø)
src/sentry/sentry_metrics/querying/data/utils.py	53.84% <ø> (-4.05%)	:arrow_down:
.../sentry_metrics/querying/data_v2/transformation.py	98.09% <100.00%> (ø)
src/sentry/testutils/cases.py	85.90% <ø> (+0.05%)	:arrow_up:
src/sentry/snuba/metrics/datasource.py	95.89% <91.66%> (-0.32%)	:arrow_down:
src/sentry/snuba/metrics/utils.py	95.55% <77.77%> (-1.27%)	:arrow_down:
src/sentry/sentry_metrics/querying/visitors.py	71.15% <81.25%> (+0.85%)	:arrow_up:

... and 8 files with indirect coverage changes

[iambriccardo]

For now, we will do this only in metrics/data but I will follow up with an analogous PR on the metrics/query endpoint.