Subscriptions: Respect DisableAssetWebsocketSupport for assets (#1693)

* Bitmex: Add index asset to test config

* Subscriptions: Fix unsupported WS assets included

fixed 1692

* Bitmex: Assertify and fix tests

* Subscriptions: Fix all asset subs erroring on no enabled assets

Only subs with Empty asset should do anything when assets are empty.
If asset is set to all and no assets are enabled, we should return
nothing

testdata/configtest.json

@@ -881,6 +881,18 @@
       "configFormat": {
        "uppercase": true
       }
+     },
+     "index": {
+      "assetEnabled": true,
+      "enabled": "-BWBTC",
+      "available": "-BWBTC",
+      "requestFormat": {
+       "uppercase": true
+      },
+      "configFormat": {
+       "uppercase": true,
+       "delimiter": "-"
+      }
      }
     }
    },