Allow the email address to be cleared via the Provisioning API

[phil-davis]

Description

The Provisioning API would not allow the email address of a user to be cleared (set to the empty string - see linked issue). This PR allows the email address to be set to the empty string, so that email addresses can be cleared by suitably-privileged users.

Note: this will also be handy to have for acceptance tests - it will be possible to easily and programmatically reset a test user email address back to its "default" value, which is "no email address"

Related Issue

Part of #37424

How Has This Been Tested?

CI and local curl requests to the Provisioning API:

$ curl -X PUT http://phil:[email protected]:8080/ocs/v2.php/cloud/users/phil -d key="email" -d value="[email protected]"
<?xml version="1.0"?>
<ocs>
 <meta>
  <status>ok</status>
  <statuscode>200</statuscode>
  <message/>
 </meta>
 <data/>
</ocs>
$ php occ user:list -a email
  - admin: [email protected]
  - phil: [email protected]
$ curl -X PUT http://phil:[email protected]:8080/ocs/v2.php/cloud/users/phil -d key="email" -d value=""
<?xml version="1.0"?>
<ocs>
 <meta>
  <status>ok</status>
  <statuscode>200</statuscode>
  <message/>
 </meta>
 <data/>
</ocs>
$ php occ user:list -a email
  - admin: [email protected]
  - phil: 

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Database schema changes (next release will require increase of minor version instead of patch)
• Breaking change (fix or feature that would cause existing functionality to change)
• Technical debt
• Tests only (no source changes)

Checklist:

• Code changes
• Unit tests added
• Acceptance tests added
• Documentation ticket raised:
• Changelog item, see TEMPLATE

[codecov]

Codecov Report

Merging #37427 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##             master   #37427   +/-   ##
=========================================
  Coverage     64.61%   64.61%           
- Complexity    19254    19255    +1     
=========================================
  Files          1270     1270           
  Lines         75262    75263    +1     
  Branches       1331     1331           
=========================================
+ Hits          48628    48629    +1     
  Misses        26242    26242           
  Partials        392      392           

Flag	Coverage Δ	Complexity Δ
#javascript	54.14% <ø> (ø)	0.00 <ø> (ø)
#phpunit	65.77% <100.00%> (+<0.01%)	19255.00 <0.00> (+1.00)

Impacted Files	Coverage Δ	Complexity Δ
apps/provisioning_api/lib/Users.php	90.57% <100.00%> (+0.03%)	123.00 <0.00> (+1.00)

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