The isInf() and isNan() functions of lib.float can now handle values of None.

If None is encountered, the functions simply return False.
The model-free optimisation code now handles minfx returning nothing.

This is due to the fix of bug #21001 (https://gna.org/bugs/?21001) in relax, which is really a fix
for an upstream minfx (https://gna.org/projects/minfx/) bug (bug #21090 at
https://gna.org/bugs/?21090).