loads("") raises AttributeError instead of ValidationError

[ryanhill1]

Summary

pyqasm.loads("") raises AttributeError: 'NoneType' object has no attribute 'line' instead of a ValidationError. Whitespace-only input does the same. Every other unparseable string raises ValidationError("Failed to parse OpenQASM string: ..."), so a caller that catches PyQasmError to reject bad input lets this one through as a crash.

Repro (pyqasm 1.2.1, openqasm3 1.0.1, Python 3.12)

import pyqasm
pyqasm.loads("")        # AttributeError: 'NoneType' object has no attribute 'line'
pyqasm.loads("   \n")   # same
pyqasm.loads("this is not qasm")  # ValidationError, as expected

Expected

ValidationError, like any other program the parser cannot read. loads("OPENQASM 3.0;\n") already returns an empty module, so the empty-string case is the only one that is neither a module nor a ValidationError.

Related: #388 covers bare expression statements reaching the same AttributeError; this one is the empty / whitespace-only input.