Do not create default wallet

No longer create a default wallet. The default wallet will still be
loaded if it exists and not other wallets were specified (anywhere,
including settings.json, bitcoin.conf, and command line).

Tests are updated to be started with -wallet= if they need the default
wallet.

Added test to wallet_startup.py testing that no default wallet is
created and that it is loaded if it exists and no other wallets were
specified.

test/functional/wallet_startup.py

@@ -26,6 +26,16 @@ class WalletStartupTest(BitcoinTestFramework):
         self.start_nodes()
 
     def run_test(self):
+        self.log.info('Should start without any wallets')
+        assert_equal(self.nodes[0].listwallets(), [])
+        assert_equal(self.nodes[0].listwalletdir(), {'wallets': []})
+
+        self.log.info('New default wallet should load by default when there are no other wallets')
+        self.nodes[0].createwallet(wallet_name='', load_on_startup=False)
+        self.restart_node(0)
+        assert_equal(self.nodes[0].listwallets(), [''])
+
+        self.log.info('Test load on startup behavior')
         self.nodes[0].createwallet(wallet_name='w0', load_on_startup=True)
         self.nodes[0].createwallet(wallet_name='w1', load_on_startup=False)
         self.nodes[0].createwallet(wallet_name='w2', load_on_startup=True)