Fix NullPointerException in MsAzurePatchComplexValueRebuilder for empty String

[stefan-wauters]

Fixes #652

Fixes NullPointerException when setting complex object to an empty string with MsAzureWorkaround enabled for Patch Configuration:

{
  "schemas": ["urn:ietf:params:scim:schemas:extension:enterprise:2.0:User"],
  "Operations": {
    "op": "replace",
    "path": "manager",
    "value": ""
  }
}

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 86.78%. Comparing base (598fa37) to head (00ca6c7).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #653 +/- ## ========================================= Coverage 86.78% 86.78% Complexity 580 580 ========================================= Files 251 251 Lines 9659 9659 Branches 1213 1213 ========================================= Hits 8383 8383 Misses 889 889 Partials 387 387 ```

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