fix: assign button grey out

[jesperhodge]

Description

On the Group assignment modal, when entering email addresses and then deleting them with backspace, the assign button stayed grayed out. This fixes it.

Links

JIRA ticket
video

Testing instructions

• Go to an enterprise with existing Learner Groups and existing courses
• Go to Leaner Credit Management
• Click on the "catalog" tab
• Click on "Assign" and select a date -> The Assign modal should open
• The Assign button on the bottom should be disabled
• assign a group via the drop down
• The assign button should be enabled
• Enter an email address in the text field
• Delete the email from the text field via backspace again
• The assign button should still be enabled

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 86.46%. Comparing base (1010d7d) to head (4419d25).
Report is 8 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1418      +/-   ##
==========================================
+ Coverage   86.44%   86.46%   +0.02%     
==========================================
  Files         660      660              
  Lines       14923    14921       -2     
  Branches     3167     3159       -8     
==========================================
+ Hits        12900    12902       +2     
+ Misses       1951     1949       -2     
+ Partials       72       70       -2     

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