uci:get_list: always return a table, no matter what.

Previously, get_list("fake", "non-existent", "notreal") would still
return a table, just empty.  This is nice, as you can always iterate the
returned table, without having to check it first.

However, if you happened to pass a nil for any of the parameters, you
would actually get a nil in return.  This was inconsistent.

The documentation is updated to clarify the behaviour of this function.

Signed-off-by: Karl Palsson <karlp@remake.is>

modules/luci-base/luasrc/model/uci.lua

@@ -111,7 +111,7 @@ function Cursor.get_list(self, config, section, option)
 		local val = self:get(config, section, option)
 		return ( type(val) == "table" and val or { val } )
 	end
-	return nil
+	return {}
 end
 
 function Cursor.get_first(self, conf, stype, opt, def)

modules/luci-base/luasrc/model/uci.luadoc

@@ -88,7 +88,8 @@ Get an option or list and return values as table.
 @param config	UCI config
 @param section	UCI section name
 @param option	UCI option
-@return			UCI value
+@return		table.  If the option was not found, you will simply get
+--		an empty table.
 ]]
 
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