Add test coverage for missing general_calculations in MolecularData.load (#1251)

This change removes the `pragma: nocover` and the associated TODO in
`src/openfermion/chem/molecular_data.py` by adding a test case in
`src/openfermion/chem/molecular_data_test.py` that triggers the branch
where `general_calculations_keys` or `general_calculations_values` are
missing from the HDF5 file.

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>

Author: mhucka

src/openfermion/chem/molecular_data.py

@@ -909,11 +909,10 @@ def load(self):
                         key.tobytes().decode('utf-8'): value
                         for key, value in zip(keys[...], values[...])
                     }
+                else:
+                    self.general_calculations = {}
             else:
-                # TODO: test the no cover
-                # no coverage here because pathway is check on
-                # bad user generated file
-                self.general_calculations = None  # pragma: nocover
+                self.general_calculations = {}
 
     def get_from_file(self, property_name):
         """Helper routine to re-open HDF5 file and pull out single property.

src/openfermion/chem/molecular_data_test.py

@@ -12,7 +12,9 @@
 """Tests for molecular_data."""
 
 import os
+import tempfile
 import unittest
+import h5py
 import numpy.random
 import scipy.linalg
 import numpy as np
@@ -347,3 +349,21 @@ def test_missing_calcs_for_integrals(self):
             molecule.get_k()
         with self.assertRaises(MissingCalculationError):
             molecule.get_antisym()
+
+    def test_load_no_general_calculations(self):
+        # Make fake molecule.
+        with tempfile.NamedTemporaryFile(suffix='.hdf5') as tmp_file:
+            filename = tmp_file.name
+            geometry = [('H', (0.0, 0.0, 0.0)), ('H', (0.0, 0.0, 0.7414))]
+            basis = '6-31g*'
+            multiplicity = 1
+            molecule = MolecularData(geometry, basis, multiplicity, filename=filename)
+            molecule.save()
+
+            # Manually remove the general_calculations_keys dataset.
+            with h5py.File(filename, 'r+') as f:
+                del f['general_calculations_keys']
+
+            # Load the molecule and check that general_calculations is empty.
+            new_molecule = MolecularData(filename=filename)
+            self.assertEqual(new_molecule.general_calculations, {})