multi: Inbound fees are retained when not provided

Fixes the problem that inbound base fee and fee rate are overwritten
with 0 if they are not specified in PolicyUpdateRequest. This ensures
backward compatibility with older rpc clients that do not yet support
the inbound feature.

lnrpc/lightning.swagger.json

@@ -5297,6 +5297,21 @@
         }
       }
     },
+    "lnrpcInboundFee": {
+      "type": "object",
+      "properties": {
+        "base_fee_msat": {
+          "type": "integer",
+          "format": "int32",
+          "description": "The inbound base fee charged regardless of the number of milli-satoshis\nreceived in the channel. By default, only negative values are accepted."
+        },
+        "fee_rate_ppm": {
+          "type": "integer",
+          "format": "int32",
+          "description": "The effective inbound fee rate in micro-satoshis (parts per million).\nBy default, only negative values are accepted."
+        }
+      }
+    },
     "lnrpcInitiator": {
       "type": "string",
       "enum": [
@@ -6585,13 +6600,9 @@
           "type": "boolean",
           "description": "If true, min_htlc_msat is applied."
         },
-        "inbound_base_fee_msat": {
-          "type": "integer",
-          "format": "int32"
-        },
-        "inbound_fee_rate_ppm": {
-          "type": "integer",
-          "format": "int32"
+        "inbound_fee": {
+          "$ref": "#/definitions/lnrpcInboundFee",
+          "description": "Optional inbound fee. If unset, the previously set value will be\nretained [EXPERIMENTAL]."
         }
       }
     },