Make device parameter units/idx optional

[dstrain115]

• Currently, the device parameters are picking up zero as the index and failing.
• Need to make them optional so we can set this to None when it is not an array index.
• Yay for proto3!

[CirqBot]

Automerge cancelled: A required status check is not present.

Missing statuses: ['Typescript tests coverage']

[codecov]

Codecov Report

Patch coverage: 90.78% and project coverage change: -0.01% ⚠️

Comparison is base (ed26d2f) 97.60% compared to head (6beaaf8) 97.60%.
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6264      +/-   ##
==========================================
- Coverage   97.60%   97.60%   -0.01%     
==========================================
  Files        1116     1116              
  Lines       95848    95912      +64     
==========================================
+ Hits        93556    93615      +59     
- Misses       2292     2297       +5     

Files Changed	Coverage Δ
cirq-google/cirq_google/api/v2/sweeps.py	100.00% <ø> (ø)
...rq_google/cloud/quantum_v1alpha1/types/__init__.py	100.00% <ø> (ø)
cirq-google/cirq_google/api/v2/run_context_pb2.py	35.48% <12.50%> (ø)
...q-core/cirq/sim/density_matrix_simulation_state.py	99.02% <100.00%> (+0.10%)	⬆️
cirq-core/cirq/sim/simulation_state_test.py	98.51% <100.00%> (ø)
cirq-google/cirq_google/api/v2/sweeps_test.py	100.00% <100.00%> (ø)
cirq-google/cirq_google/cloud/quantum/__init__.py	100.00% <100.00%> (ø)
...gle/cirq_google/cloud/quantum_v1alpha1/__init__.py	100.00% <100.00%> (ø)
...irq_google/cloud/quantum_v1alpha1/types/quantum.py	100.00% <100.00%> (ø)
cirq-google/cirq_google/engine/engine_client.py	100.00% <100.00%> (ø)
... and 1 more

... and 2 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.