BUG: Fix bug when creating model from formula with missing data

[bashtage]

Fix bug
Fix docstring examples

[codecov]

Codecov Report

Merging #438 (48bb753) into main (2850e0d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #438   +/-   ##
=======================================
  Coverage   99.53%   99.53%           
=======================================
  Files          98       98           
  Lines       16914    16914           
  Branches     1989     1989           
=======================================
  Hits        16835    16835           
  Misses         39       39           
  Partials       40       40           

Impacted Files	Coverage Δ
linearmodels/iv/_utility.py	100.00% <ø> (ø)
linearmodels/iv/absorbing.py	98.73% <ø> (ø)
linearmodels/iv/covariance.py	99.06% <ø> (ø)
linearmodels/iv/results.py	99.23% <ø> (ø)
linearmodels/panel/model.py	99.32% <ø> (ø)
linearmodels/panel/results.py	99.75% <ø> (ø)

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