Fix NullPointerException on addErrorCode when input is Invalid error code

Problem:
1. When the errocode is not defined in DhcpErrorCode enum.
   Then the DhcpErrorCode.forNumber(errorCode) will return null.

2. Then use null as the parameter cause NullPointerException on
   addErrorCode.
   (Because the addErrorCode need nonNullable parameter)

Solution:
If the errorcode is not defined in the Dhcp ErrorCode enumeration,
please use ET_UNKNOWN instead as AddErrorCode parameter, not null.

Bug: 151796056
Test: atest
Change-Id: I43b8415ab98b4cf6520c3240dc544a9f62730a4e