The default name is more commonly returned on code such as:
x, y = Signal(), Signal()
The case where the opcode is not recognized is only encountered
when older Amaranth is ran on a newer Python interpreter (with more
opcodes).
Returning None instead of a name here caused issues in the RTLIL
backend, which would incorrectly use $\d+ names for ports, since
the RTLIL backend assumed the name of a signal is always a string.
Fixes#733.
aaec7e0d27