Handle handle empty figures in tight_layout (matplotlib#8891)

* Fix tight_layout to handle empty figures

* Fix formatting

* Change tight_layout to always return a dictionary

Author: Molly Rossow

lib/matplotlib/tests/test_tightlayout.py

@@ -264,3 +264,12 @@ def _subplots():
                 child.set_visible(False)
 
     plt.tight_layout()
+
+
+def test_empty_layout():
+    """Tests that tight layout doesn't cause an error when there are
+    no axes.
+    """
+
+    fig = plt.gcf()
+    fig.tight_layout()

lib/matplotlib/tight_layout.py

@@ -319,6 +319,9 @@ def get_tight_layout_figure(fig, axes_list, subplotspec_list, renderer,
 
         subplots.append(ax)
 
+    if (len(nrows_list) == 0) or (len(ncols_list) == 0):
+        return {}
+
     max_nrows = max(nrows_list)
     max_ncols = max(ncols_list)