Add default None arg to ComponentGraph.instantiate

[bchen1116]

fix #2698

[codecov]

Codecov Report

Merging #2796 (e682e46) into main (c0ed2ba) will increase coverage by 0.1%.
The diff coverage is 100.0%.

@@           Coverage Diff           @@
##            main   #2796     +/-   ##
=======================================
+ Coverage   99.8%   99.8%   +0.1%     
=======================================
  Files        297     297             
  Lines      27715   27718      +3     
=======================================
+ Hits       27647   27650      +3     
  Misses        68      68             

Impacted Files	Coverage Δ
evalml/pipelines/component_graph.py	99.8% <100.0%> (ø)
...valml/tests/pipeline_tests/test_component_graph.py	100.0% <100.0%> (ø)

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[angela97lin]

Love it! Thank you 😁