resource_channels.c: Don't call ast_channel_get_by_name on empty optional arguments

resource_channels.c: Don't call ast_channel_get_by_name on empty optional arguments

`ast_ari_channels_create` and `ast_ari_channels_dial` called the
`ast_channel_get_by_name` function with optional arguments. Since
8f1982c4d6, this function logs an error for empty channel names.
This commit adds checks for empty optional arguments that are used
to call `ast_channel_get_by_name` to prevent these error logs.