If one loads a wallet via RPC (`loadwallet w2`), then select w2, select back to the default wallet (which is an empty string), that default wallet cannot be access through the RPC console because the current code only points to the wallet endpoint if the wallet name is not empty.

This is a quick fix that reenables accessing the default wallet in case an additional wallet has been loaded.

Using "" for the default wallet may not be ideal in other cases and it may make more sense to change it at a deeper level (wallet.cpp). See discussion here which where the reasons for the current behaviour in master:
https://github.com/bitcoin/bitcoin/pull/11687#issuecomment-370862718

@jnewbery @promag @ryanofsky

arc patch D4236
arc patch D4245 (this patch)
make check
./bitcoin-qt
Help -> Debug -> Console
loadwallet ""

createwallet test
Make sure the default wallet is selected
getwalletinfo

Wallet file not specified (must request wallet RPC through /wallet/<filename> uri-path).  (code -19)