expression migrator generates invalid style if icon-image value is a categorical function

[samanpwbb]

mapbox-gl-js version: latest 1.x.x

browser: N/A

Steps to Trigger Behavior

1. Attempt to migrate a style that uses a categorical function on the text-image field.
2. See validation error.

Link to Demonstration

#10055

Expected Behavior

Migrated style is valid. I see two ways to fix this problem:

1. Make migrator insert an empty string as fallback value for icon-image when migrating categorical expression.
2. Make GL JS support undefined values for icon-image.

I can take care of fixing this if someone on GL JS has a preferred approach between those two. I'm thinking using an empty string as fallback is the best option. This can be fixed by adjusting the logic around categorical conversion for image type values:

mapbox-gl-js/src/style-spec/function/convert.js

Line 140 in ff8e087

expression.push(convertLiteral(coalesce(parameters.default, propertySpec.default)));

– this line can resolve to undefined, leading to an invalid style.

Actual Behavior

Migrated style is not valid.

// Input
{
  base: 1,
  type: 'categorical',
  property: 'type',
  stops: [['park', 'some-icon']]
}

// Output
[ 'match', [ 'get', 'type' ], 'park', 'some-icon', undefined ]

Error: "layers[0].layout.icon-image[4]: 'undefined' value invalid. Use null instead."