Fix PytestUnraisableExceptionWarning with cmd.fragment()

[speleo3]

pytest reports PytestUnraisableExceptionWarning in tests/api/build.py::TestNucBuilder.

Minimal example is to call cmd.fragment("gtp").

$ python -c 'from pymol import cmd;cmd.fragment("gtp")'
Exception ignored in PyObject_HasAttrString(); consider using PyObject_HasAttrStringWithError(), PyObject_GetOptionalAttrString() or PyObject_GetAttrString():
Traceback (most recent call last):
  File "/d2/bld/env-open-source-3-1-master/lib/python3.13/site-packages/pymol/importing.py", line 205, in load_object
    r = _cmd.load_object(_self._COb,str(name),object,int(state)-1,
AttributeError: 'Indexed' object has no attribute 'molecule'

Since 'molecule' seems to be an optional member of the chempy Indexed object, the AttributeError can be deliberately ignored.