Fix empties assertions and docstring in discrete space tests (mesa#3793)

Complete test_empties_space by asserting empties count after adding and removing agents to discrete spaces.

Fix copy-pasted docstring in test_agents_property.

Co-authored-by: Jan Kwakkel <j.h.kwakkel@tudelft.nl>

Author: Dilraj07

tests/discrete_space/test_discrete_space.py

@@ -555,8 +555,17 @@ def test_empties_space():
     assert len(grid.empties) == n
 
     model = Model()
+    agents = []
     for i in range(8):
-        grid._cells[i].add_agent(CellAgent(model))
+        agent = CellAgent(model)
+        grid._cells[i].add_agent(agent)
+        agents.append(agent)
+
+    assert len(grid.empties) == n - 8
+
+    # Remove an agent and verify empties count updates
+    grid._cells[0].remove_agent(agents[0])
+    assert len(grid.empties) == n - 7
 
 
 def test_cell_missing_exception():
@@ -604,7 +613,7 @@ def test_grid_validate_parameters():
 
 
 def test_agents_property():
-    """Test empties method for Discrete Spaces."""
+    """Test agents property for Discrete Spaces."""
     n = 10
     m = 20
     rng = 42