public static Callback ifNull(Callback c) {
		return c == null ? callback() : c;
	}
Returns the given callback object if it is not {@code null}.  Otherwise, a {@link #callback()}
is returned.  This is useful to avoid using {@code null}.

@param c the callback function to check for {@code null}
@return a non-null callback function
Ghidra/Framework/Utility/src/main/java/utility/function/Dummy.java