packet: don't always skip empty values in .command() (#4238)

This patch fixes an issue where empty lists assigned explicitly to
override non-empty default values weren't present in commands generated
by Packet.command and the packets were different.

For example `DNS(qd=[]).command()` generated `DNS()` and it turned into
packets with the default "example.com IN A" query. With this patch
applied it generates `DNS(qd=[])`.

scapy/packet.py

@@ -1693,7 +1693,7 @@ def command(self):
         f = []
         for fn, fv in self.fields.items():
             fld = self.get_field(fn)
-            if isinstance(fv, (list, dict, set)) and len(fv) == 0:
+            if isinstance(fv, (list, dict, set)) and not fv and not fld.default:
                 continue
             if isinstance(fv, Packet):
                 fv = fv.command()

test/scapy/layers/dns.uts

@@ -414,3 +414,11 @@ assert pkt.an.rdata == b'140C768FFE28@Freebox Server._raop._tcp.local.'
 pkt = DNS(qr=1, qd=None, aa=1, rd=1)
 pkt = DNS(bytes(pkt))
 assert pkt.qd == []
+
+= DNS - command
+
+p = DNS()
+assert p == eval(p.command())
+
+p = DNS(qd=[])
+assert p == eval(p.command())