don't fit a GLM when fixed effects are empty

[palday]

Thanks for contributing!

Did behavior change? Did you add need features? If so, please update NEWS.md

• add entry in NEWS.md
• after opening this PR, add a reference and run docs/NEWS-update.jl to update the cross-references.

Should we release your changes right away?

No, should be bundled in #639

If so, bump the version:

• I've bumped the version appropriately

[codecov]

Codecov Report

Base: 94.23% // Head: 94.24% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (072a3e9) compared to base (2c041e5).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 072a3e9 differs from pull request most recent head 264b194. Consider uploading reports for the commit 264b194 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #657   +/-   ##
=======================================
  Coverage   94.23%   94.24%           
=======================================
  Files          29       29           
  Lines        2795     2796    +1     
=======================================
+ Hits         2634     2635    +1     
  Misses        161      161           

Impacted Files	Coverage Δ
src/generalizedlinearmixedmodel.jl	90.54% <100.00%> (+0.02%)	⬆️

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[dmbates]

Thanks for looking into this.