Fix holdout warning message showing when using default parameters

[christopherbunn]

Currently, we show our warning that AutoML search will not be evaluated with a holdout set even when holdout_set_size=0 (which is the default). This PR fixes this and a typo in the error message.

[codecov]

Codecov Report

Merging #3727 (ae7abee) into main (4c0f57a) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #3727     +/-   ##
=======================================
+ Coverage   99.7%   99.7%   +0.1%     
=======================================
  Files        340     340             
  Lines      34667   34671      +4     
=======================================
+ Hits       34535   34539      +4     
  Misses       132     132             

Impacted Files	Coverage Δ
evalml/automl/automl_search.py	99.5% <100.0%> (ø)
evalml/tests/automl_tests/test_automl.py	99.5% <100.0%> (+0.1%)	⬆️

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[eccabay]

Looks great!