[select] fix(Omnibar): handle initialContent undefined value correctly

[DannyDelott]

Checklist

• Includes tests

Changes proposed in this pull request:

Fixes a bug where Omnibar is unable to render all items when initialContent is explicitly set to undefined.

Related to #3174

Reviewers should focus on:

The documentation for Omnibar's initialContent states:

If omitted, all items will be rendered (or result of itemListPredicate with empty query).
If explicit null, nothing will be rendered when query is empty.

Under the hood, QueryList is only checking undefined:

blueprint/packages/select/src/common/itemListRenderer.ts

Lines 84 to 86 in 9bce0cd

	if (props.query.length === 0 && initialContent !== undefined) {
	return initialContent;
	}

It's probably a larger breaking change to remove the default null value, but this at least solves the problem in the near term, ie:

<Omnibar
  // render all the items when no query is present
  initialContent={undefined}
  isOpen={...}
  items={[...]}
  itemRenderer={...}
  onItemSelect={...}
  />

[adidahiya]

docs preview build

[adidahiya]

thanks @DannyDelott