test: Don't send empty named args with cli

If python passed None for an optional (i.e. 'null' is
sent), this will lead to the arg being interpreted as not
provided by bitcoind - except for string args, for which the arg is
interpreted as as 'null' string. Bypass this by not sending
named args to bitcoin-cli - so that the default value will
actually be used.

Also drops an unnecessary str() conversion, kwargs keys
are always strings.

test/functional/test_framework/test_node.py

@@ -918,7 +918,7 @@ class TestNodeCLI():
     def send_cli(self, clicommand=None, *args, **kwargs):
         """Run bitcoin-cli command. Deserializes returned string as python object."""
         pos_args = [arg_to_cli(arg) for arg in args]
-        named_args = [str(key) + "=" + arg_to_cli(value) for (key, value) in kwargs.items()]
+        named_args = [key + "=" + arg_to_cli(value) for (key, value) in kwargs.items() if value is not None]
         p_args = self.binaries.rpc_argv() + [f"-datadir={self.datadir}"] + self.options
         if named_args:
             p_args += ["-named"]

test/functional/wallet_address_types.py

@@ -78,7 +78,6 @@ class AddressTypeTest(BitcoinTestFramework):
         ]
         # whitelist peers to speed up tx relay / mempool sync
         self.noban_tx_relay = True
-        self.supports_cli = False
 
     def skip_test_if_missing_module(self):
         self.skip_if_no_wallet()