Fixed issue MDAnalysis#2915 and added a pytest to demonstrate the fix…

…. Sphzone operating on an empty selection now returns an empty atom group.

package/MDAnalysis/core/selection.py

@@ -375,11 +375,13 @@ def __init__(self, parser, tokens):
         self.sel = parser.parse_expression(self.precedence)
 
     @return_empty_on_apply
-    def apply(self, group):
+    def apply(self, group): # TODO fix atom selection
         indices = []
         sel = self.sel.apply(group)
         box = self.validate_dimensions(group.dimensions)
         periodic = box is not None
+        if len(sel) == 0:
+            return group[[]]
         ref = sel.center_of_geometry().reshape(1, 3).astype(np.float32)
         pairs = distances.capped_distance(ref, group.positions, self.cutoff,
                                           box=box,

testsuite/MDAnalysisTests/core/test_atomselections.py

@@ -730,6 +730,11 @@ def test_sphzone(self, u, periodic, expected):
 
         assert len(sel) == expected
 
+    @pytest.mark.parametrize('periodic,expected', ([True, 33], [False, 25]))
+    def test_sphzone(self, u, periodic, expected):
+        sel = u.select_atoms('sphzone 5.0 name NOT_A_NAME', periodic=periodic)
+        assert len(sel) == 0
+
 
 class TestTriclinicDistanceSelections(BaseDistanceSelection):
     @pytest.fixture()