fix(select): skip the divider children in getDisplay

[boaz0]

What: Closes #4945

//cc @jenny-s51

[patternfly-build]

PF4 preview: https://patternfly-react-pr-4998.surge.sh
A11y report: https://patternfly-react-pr-4998-a11y.surge.sh

[codecov-io]

Codecov Report

Merging #4998 into master will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4998   +/-   ##
=======================================
  Coverage   52.17%   52.17%           
=======================================
  Files         529      529           
  Lines        9718     9718           
  Branches     3580     3581    +1     
=======================================
  Hits         5070     5070           
  Misses       3995     3995           
  Partials      653      653           

Flag	Coverage Δ
#patternfly4	52.17% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...ckages/react-core/src/components/Select/Select.tsx	41.27% <100.00%> (ø)

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[nicolethoen]

LGTM - wise to add the new test

[tlabaj]

LGTM

[boaz0]

Thank you all: @nicolethoen @dlabrecq and @tlabaj for reviewing!