fix(Select): select clear button

[pedrokohler]

Context

The ohif/ui select does not work if you try to clear the selection. The onChange function throws an error because it tries to select the length property of null:

Changes & Results

Testing

Checklist

PR

• [] My Pull Request title is descriptive, accurate and follows the
  semantic-release format and guidelines.

Code

• [] My code has been well-documented (function documentation, inline comments,
  etc.)

Public Documentation Updates

• [] The documentation page has been updated as necessary for any public API
  additions or removals.

Tested Environment

• [] OS:
• [] Node version:
• [] Browser:

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