rpcserver: error on zero local balance in openchan req

In this commit, we catch and error on the specific case where an
openchannel request has a zero-value local funding amount. This is just
to ensure that the error message returned makes more sense.

docs/release-notes/release-notes-0.16.0.md

@@ -27,6 +27,9 @@ transaction](https://github.com/lightningnetwork/lnd/pull/6730).
   method to the RPC to allow subscribing to updates from any inflight payment.
   Similar to TrackPaymentV2, but for any inflight payment.
 
+* [Catch and throw an error](https://github.com/lightningnetwork/lnd/pull/6945)
+  during `openchannel` if the local funding amount given is zero. 
+
 ## Wallet
 
 * [Allows Taproot public keys and tap scripts to be imported as watch-only