lua: don't require key for mp.add_key_binding()

Requested. The intention is that scripts can provide mappable actions
for key bindings without setting a default key.

DOCS/man/lua.rst

@@ -192,7 +192,8 @@ The ``mp`` module is preloaded, although it can be loaded manually with
     Register callback to be run on a key binding. The binding will be mapped to
     the given ``key``, which is a string describing the physical key. This uses
     the same key names as in input.conf, and also allows combinations
-    (e.g. ``ctrl+a``).
+    (e.g. ``ctrl+a``). If the key is empty or ``nil``, no physical key is
+    registered, but the user still can create own bindings (see below).
 
     After calling this function, key presses will cause the function ``fn`` to
     be called (unless the user remapped the key with another binding).

player/lua/defaults.lua

@@ -145,7 +145,7 @@ local function update_key_bindings()
         end
         local cfg = ""
         for k, v in pairs(key_bindings) do
-            if v.forced ~= def then
+            if v.bind and v.forced ~= def then
                 cfg = cfg .. v.bind .. "\n"
             end
         end
@@ -161,7 +161,6 @@ local function add_binding(attrs, key, name, fn, rp)
         fn = name
         name = reserve_binding()
     end
-    local bind = key
     local repeatable = rp == "repeatable" or rp["repeatable"]
     if rp["forced"] then
         attrs.forced = true
@@ -205,7 +204,9 @@ local function add_binding(attrs, key, name, fn, rp)
         end
         msg_cb = fn
     end
-    attrs.bind = bind .. " script-binding " .. mp.script_name .. "/" .. name
+    if key and #key > 0 then
+        attrs.bind = key .. " script-binding " .. mp.script_name .. "/" .. name
+    end
     attrs.name = name
     key_bindings[name] = attrs
     update_key_bindings()