Consider making `ClaimAccessor#getClaimXXX` methods behave consistently if claim value is `null`

This ticket is created based on this comment.

Expected Behavior
ClaimAccessor#getClaim() and ClaimAccessor#getClaimAsString() should behave the same way as other ClaimAccessor#getClaimAsXXX methods.

There are few options to consider:

• all ClaimAccessor#getClaimAsXXX methods will throw NullPointerException if claim value is null and claim value is converted to null. I suppose, this is a preferred way.
• all ClaimAccessor#getClaimAsXXX methods will throw IllegalArgumentException without claim value class, if claim value is null and claim value is converted to null (no NullPointerException)
• null will be valid return value even if converter converts claim value to null (no assertion). This was discussed here
• ...

Current Behavior
All ClaimAccessor#getClaimAsXXX methods (except ClaimAccessor#getClaim() and ClaimAccessor#getClaimAsString()) throw NullPointerException if claim value is null and claim value is converted to null.

Exception is thrown when message for IllegalArgumentException is evaluated:

Assert.isTrue(convertedValue != null,
				() -> "Unable to convert claim '" + claim + "' of type '" + claimValue.getClass() + "' to XXX.");

On the other side,

• ClaimAccessor#getClaim() returns null if claim value is null.
• ClaimAccessor#getClaimAsString() returns null if claim value is converted to null.

Start by reading the ClaimAccessor#getClaim(), getClaimAsString(), and other getClaimAsXXX methods to compare their null handling. Review the linked pull-request comment and issue discussion before choosing a consistent behavior. Done means the selected behavior is applied consistently across these accessors and verified for null claim values.