@Override
	public final boolean isNull() {
		return isNull;
	}
Construct a fixed-length field.  A null "state" may only be established 
by invoking the {@link #setNull()} method after construction provided
the instance is mutable.
@param data initial storage value (may be null)
@param immutable true if field value is immutable
Ghidra/Framework/DB/src/main/java/db/FixedField.java