Given that registers are available when a value has been supplied,
it's logical to assume (a priori) that a register is unavailable
when no value has been supplied. A register's valid "bit" allows
for this since there are 2 states that indicate unavailability:
One that indicates a temporary state (0) and one that indicates a
permanent state (-1). The initial state of a register is the temporarily
unavailable state, which triggers fetching the register from the
target. The target can change the state to permanently unavailable
or supply the value (it can also, theoretically at least, leave the
state unmodified and not provide a value). Hence, the a priori
assumption that registers are unavailable when no value has been
supplied (i.e. when the valid "bit" is not 1) seems to yield good
behaviour when implemented as such. I would say then that gdb knows
when a value is not available.