decoder: Accept more forms of "unconditional wait()" (None, no args)

The Decoder.wait() method expects a list of dicts, or as a special form
a single dict, to specify the conditions to wait for. An empty dict or
an empty list mean "unconditional wait", requesting the very next sample.

Accept None as well as no arguments at all in Decoder.wait() calls. This
shall better reflect the intent and slightly unobfuscate PD code, as well
as avoid creation of potentially expensive Python objects at the call site.