[ML] Function selection dropdown does not open on arrow-click

[pheyos]

Found in version

• 6.4 build 1362

Browser

• Firefox

Steps to reproduce

• Create a new job with the multi metric wizard or the population wizard
• In the function selection dropdown click exactly on the black down-arrow button (easier when zoomed in a bit)
  

Expected result

• The dropdown opens

Actual result

• The dropdown doesn't open

Additional information

• When clicking somewhere else in the function field the dropdown opens
• This is probably caused by [ML] Fixes agg-type dropdowns size and styling. #19816

[elasticmachine]

Pinging @elastic/ml-ui

[jgowdyelastic]

Related to #19816