Trap divide-by-zero in jump step

[mwregan2]

This PR addresses a bug found in DMS B9.3 testing, such that when there are no usable groups, the code calculating the number of CRs per second divides by zero.

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (041c730) 76.56% compared to head (08b5b6d) 76.56%.

❗ Current head 08b5b6d differs from pull request most recent head 66262f9. Consider uploading reports for the commit 66262f9 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7723   +/-   ##
=======================================
  Coverage   76.56%   76.56%           
=======================================
  Files         456      456           
  Lines       36877    36878    +1     
=======================================
+ Hits        28234    28235    +1     
  Misses       8643     8643           

Flag	Coverage Δ		*Carryforward flag
nightly	77.43% <ø> (ø)		Carriedforward from 041c730

*This pull request uses carry forward flags. Click here to find out more.

Impacted Files	Coverage Δ
jwst/jump/jump.py	100.00% <100.00%> (ø)

☔ View full report in Codecov by Sentry.
📢 Do you have feedback about the report comment? Let us know in this issue.

[hbushouse]

Looks OK

[hbushouse]

Regression test run at https://plwishmaster.stsci.edu:8081/job/RT/job/JWST-Developers-Pull-Requests/780/ has unrelated failures.