fix SchemaIO exception when onChange is missing

[imanjra]

What changes are proposed in this pull request?

Fixes an exception in SchemaIO when onChange is not provided. Regressed in #3833

How is this patch tested? If it is not, please explain why.

Using OperatorIO debugging panel

Release Notes

Is this a user-facing change that should be mentioned in the release notes?

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• Yes. Give a description of this change to be included in the release
  notes for FiftyOne users.

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What areas of FiftyOne does this PR affect?

• App: FiftyOne application changes
• Build: Build and test infrastructure changes
• Core: Core fiftyone Python library changes
• Documentation: FiftyOne documentation changes
• Other

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

❗ No coverage uploaded for pull request base (release/v0.23.0@3738536). Click here to learn what that means.

Additional details and impacted files

@@                Coverage Diff                 @@
##             release/v0.23.0    #3840   +/-   ##
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  Coverage                   ?   15.65%           
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  Files                      ?      728           
  Lines                      ?    80534           
  Branches                   ?     1074           
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  Hits                       ?    12609           
  Misses                     ?    67925           
  Partials                   ?        0           

Flag	Coverage Δ
app	15.65% <0.00%> (?)

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