FHIR primitive static factory helpers should throw appropriate exceptions when null is passed

[lmsurpre]

Is your feature request related to a problem? Please describe.
When populating an object from a different source, its convenient to use the static factory methods like Code.of() like the following

Coding.builder()
    .code(Code.of(myObj.getMyCode()))
    .display(string("Pregnant"))
    .build()

However, if that source data can contain nulls, then today this breaks on the Code.of(null) with the following:

java.lang.IllegalStateException: ele-1: All FHIR elements must have a @value or children

This despite the following code working with no issue:

Coding.builder()
    .code(null)
    .display(string("Pregnant"))
    .build()

Describe the solution you'd like
Code.of(null) and similar convience functions should return null if null is passed in. Instead of trying to create a privitive FHIR object with no value which is guaranteed to fail with ele-1.

Describe alternatives you've considered
Add javadoc to explain it will throw if you pass a null.

[lmsurpre]

Team decision:

1. update javadoc to say that the arguments to the factory methods must be non-null
2. add Object.requireNonNull to all the generated factory methods (with the name of the argument so we know what was null)

[michaelwschroeder]

I have validated the javadoc and the addition of Object.requireNotNull() method calls via visual inspection of classes in the fhir-model project and have validated that when attempting to pass a null value to these factory methods a NullPointerException is thrown indicating the name of the argument containing the null value.