Don't check the OCAT for "hot" ACIS observations if there are no observations (vehicle loads)

[jzuhone]

Description

acis_thermal_check performs a check of FP temperature violations against the OBSIDs in the science load. For vehicle-only loads there are no OBSIDs to check. The new-ish fetch_ocat_data function queries the OCAT with a list of OBSIDs, to check for observations that can go to -109 C because of a low number of expected counts.

For vehicle-only loads, the list of OBSIDs passed to fetch_ocat_data is empty, but instead of handling this properly, the function queries the OCAT and returns garbage, and BrokenPipe errors are returned to screen.

acis_thermal_check appears to proceed normally after this, but to avoid any issue this PR changes the behavior so that the OCAT is not checked if the OBSID list is empty.

Testing

• Passes unit tests on MacOS, linux, Windows (at least one required)
• Functional testing

[jazan12]

Hard to argue with this one.

[Gregg140]

Good structured programming/Software Engineering technique frowns upon multiple exits from a function or a method. I don't like/use multiple myself, but it's legal Python so I approve the change.

[jzuhone]

@Gregg140 fe4c78d 😉

[Gregg140]

I approve the update.

[jzuhone]

@Gregg140 did it fix the issues on your end? I shouldn't merge until I know that for sure (I'm also waiting on my regression tests to finish).