[Validator] The validator shouldn't report an error of missing an required element when a "data-absent-reason" extension is present

[mmsmits]

Describe the bug
The validator currently reports an error when an element with a cardinality of 1...1 is missing , however this is allowed when there is a "data-absent-reason" extension present instead.

To Reproduce
Run the RunSingleTest() test method in ValidationManifestTest.cs using allergy.json in the R4 branch for a test that checks for this.

Expected behavior
Don't throw an error on a missing element when the data-absent-reason extension is present.

Version used:

• FHIR Version: STU3, R4, R5

Additional context
This issue/feature has come to light comparing the Java validator against the .NET validator.

[mmsmits]

After investigating this.
No error is thrown when providing for a missing element, but only for not providing a code for an element that has a required binding. Just like the java validator does. No fix needed.