Forcing defaults to zero if stream is missing

[alicebarthel]

The current implementation of Forcing sets the forcing fields to zero if the code fails to read in the stream.
This is to avoid imposing a forcing file requirement to existing tests.

In the long run, the better implementation is:
e.g. if SfcStressForcingTendencyEnable is True, throw a critical error if SfcStressZonal and SfcStressMeridional are not included in the forcing stream
This would be applied to each set of forcing fields and their corresponding flags.

[katsmith133]

Is this the same issue as #440?

[alicebarthel]

Not exactly, this is a behavior that we have implemented intentionally but want to eventually iterate over. This point of this issue is to revisit it when the time comes.