Classical bitset of size zero not supported

[garrison]

If one creates a bit array of size zero (e.g. bit[0] c1;), the program will fail to parse

qe-qasm/include/qasm/AST/ASTCBit.h

Line 81 in f6d695f

assert(S > 0 && "Invalid bitset of size zero!");

However, this is not explicitly forbidden by the openqasm3 spec, as far as I can tell (see https://openqasm.com/language/types.html#arrays).

In the circuit-knitting-toolbox, we programmatically generate many circuits, all of which have two registers for consistency in handling them. But, one of the registers has zero bits in some of the circuits.

We can work around this easily, but I wanted to flag it as something that would be desirable to either support or clarify.