py2rpm: Don't crash when some manifest fields are not defined

Make py2rpm handle gracefully (instead of crashing) when authors
leave license or homepage fields unspecified in their package
manifest.

Test case: PuLP==1.5.4

Change-Id: I0b99487afae2e03df24e53e94b02ebddfd9edaf1
Mark PuLP as arch-dependent

Change-Id: I2fbe7adadf369ba0bc206a2a4e33583b49fdf4b7