feat(GlDatepicker): Allow for empty date values

What does this MR do?

This change allows the user to enter and submit an empty date as the value for GlDatepicker by hitting the enter key on an empty string. Previously the input event would not be fired for empty values. This is especially useful for pages such as the audit event logs where a null date is the default and a user was not able to reset the field after picking a date.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines.
• GitLab UI's contributing guidlines.
• If it changes a Pajamas-compliant component's look & feel, the MR has been reviewed by a UX designer.
• If it changes GitLab UI's documentation guidelines, the MR has been reviewed by a Technical Writer.
• If the MR changes a component's API, integration MR(s) have been opened in the following projects to ensure that the @gitlab/ui package can be upgraded quickly after the changes are released:
  • GitLab: mr_url
  • Customers Portal: mr_url
  • Status Page: mr_url
• Added the ~"component:*" label(s) if applicable.

Closes #789 (closed)