FIX: do not allow single element list of str in subplot_mosaic

Also check that the first element of the a list layout is not a string.

closes matplotlib#19631

Author: tacaswell

lib/matplotlib/figure.py

@@ -1692,6 +1692,8 @@ def _make_array(inp):
 
             """
             r0, *rest = inp
+            if isinstance(r0, str):
+                raise ValueError('List layout specification must be 2D')
             for j, r in enumerate(rest, start=1):
                 if isinstance(r, str):
                     raise ValueError('List layout specification must be 2D')

lib/matplotlib/tests/test_figure.py

@@ -760,6 +760,8 @@ def test_empty(self, fig_test, fig_ref, x, empty_sentinel):
     def test_fail_list_of_str(self):
         with pytest.raises(ValueError, match='must be 2D'):
             plt.subplot_mosaic(['foo', 'bar'])
+        with pytest.raises(ValueError, match='must be 2D'):
+            plt.subplot_mosaic(['foo'])
 
     @check_figures_equal(extensions=["png"])
     @pytest.mark.parametrize("subplot_kw", [{}, {"projection": "polar"}, None])