codegen: Don't add parameters with null values

Currently the methods that receive header or URL parameters add them to
the request even if the value is null. This patch changes those methods
to not add the header or URL parameter in this case.

Change-Id: I6d8b8936f219db148cff9de2825773908a289f92
Bug-Url: https://bugzilla.redhat.com/1047638
Signed-off-by: Juan Hernandez <juan.hernandez@redhat.com>