This is handled in the called function by native python routines.  The string func_name is passed to call_python_cat() function as 'uschar *name.  In the function, it does this:

  /* Identify and call appropriate function */
  pFunc = PyObject_GetAttrString(pModule, CCS name);
  if (pFunc && PyCallable_Check(pFunc))

The else of that returns a not-found error, shown here:

[exim-build@tlyons ~/projects/exim/src]$ build-Linux-i386/exim -be '${python{>,}{80}{asd}}' 2>&1 
Failed: Did not find function ''
[exim-build@tlyons ~/projects/exim/src]$ build-Linux-i386/exim -be '${python{>,   }{80}{asd}}' 2>&1 
Failed: Did not find function ''
[exim-build@tlyons ~/projects/exim/src]$ build-Linux-i386/exim -be '${python{}{80}{asd}}' 2>&1 
Failed: Did not find function ''
[exim-build@tlyons ~/projects/exim/src]$ build-Linux-i386/exim -be '${python{        }{80}{asd}}' 2>&1 
Failed: Did not find function ''

If you insist on a strcmp against US"", that's easy enough to add.