Bug-731: pcep Tlv serializers - fixed sonar issues

-no need to check for null before an insteadof
-fixed precondition checks in Tlv serializers

Change-Id: I8727f83beb713c427c0337b9058c00490631b1e8
Signed-off-by: Milos Fabian <milfabia@cisco.com>
Merge "Bug-731: pcep Tlv serializers - fixed sonar issues"