Fix issue when an empty palette is provided for an empty dataset (#3136)

* Fix issue when an empty palette is provided for an empty dataset

* Added test for empty palette with swarmplot or stripplot

Author: thuiop

seaborn/categorical.py

@@ -236,7 +236,7 @@ def _native_width(self):
     def _nested_offsets(self, width, dodge):
         """Return offsets for each hue level for dodged plots."""
         offsets = None
-        if "hue" in self.variables:
+        if "hue" in self.variables and self._hue_map.levels is not None:
             n_levels = len(self._hue_map.levels)
             if dodge:
                 each_width = width / n_levels
@@ -268,7 +268,7 @@ def plot_strips(
             jlim = 0.1
         else:
             jlim = float(jitter)
-        if "hue" in self.variables and dodge:
+        if "hue" in self.variables and dodge and self._hue_map.levels is not None:
             jlim /= len(self._hue_map.levels)
         jlim *= self._native_width
         jitterer = partial(np.random.uniform, low=-jlim, high=+jlim)

tests/test_categorical.py

@@ -2137,6 +2137,9 @@ def test_vs_catplot(self, long_df, wide_df, kwargs):
 
         assert_plots_equal(ax, g.ax)
 
+    def test_empty_palette(self):
+        self.func(x=[], y=[], hue=[], palette=[])
+
 
 class TestStripPlot(SharedScatterTests):