PUP-9294 fails to load valid 3.x function when it contains call to constant

In PUP-9294 new checking for illegal constructs in 3.x functions was introduced. Before release it was found that it crashes if the 3.x function contains a call to a constant. The one that was detected contained the following:

Float(a_string)

The logic added in PUP-9294 was not expecting calls to constants and would produce a "nil" function name thus crashing when validating the loaded 3.x function.