abacus: fix bug in collecting force (#669)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

- **Bug Fixes**
- Modified force data handling in computational simulations to return
`None` instead of an empty list when no forces are present.

- **Tests**
- Added new test scenarios to validate force data handling in labeled
system objects for computational simulations.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Signed-off-by: Jinzhe Zeng <[email protected]>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Jinzhe Zeng <[email protected]>

dpdata/abacus/scf.py

@@ -190,7 +190,7 @@ def collect_force(outlines):
 def get_force(outlines, natoms):
     force = collect_force(outlines)
     if len(force) == 0:
-        return [[]]
+        return None
     else:
         return np.array(force[-1])  # only return the last force
 
@@ -285,7 +285,7 @@ def get_frame(fname):
     data["cells"] = cell[np.newaxis, :, :]
     data["coords"] = coords[np.newaxis, :, :]
     data["energies"] = np.array(energy)[np.newaxis]
-    data["forces"] = force[np.newaxis, :, :]
+    data["forces"] = np.empty((0,)) if force is None else force[np.newaxis, :, :]
     if stress is not None:
         data["virials"] = stress[np.newaxis, :, :]
     data["orig"] = np.zeros(3)

tests/abacus.scf/INPUT.ch4-noforcestress

@@ -0,0 +1,17 @@
+INPUT_PARAMETERS
+#Parameters	(General)
+suffix 			ch4-noForceStress
+stru_file 	        STRU.ch4 #the filename of file containing atom positions
+kpoint_file        	KPT.ch4 #the name of file containing k points
+pseudo_dir		./
+nbands		  	8
+#Parameters (Accuracy)
+ecutwfc			100
+symmetry		1
+scf_nmax		50
+smearing_method		gauss #type of smearing: gauss; fd; fixed; mp; mp2; mv
+smearing_sigma		0.01
+mixing_type 		plain
+mixing_beta 		0.5
+cal_force 		1
+cal_stress 		1