[Fix issue][#3511]lineplot of empty dataframe with hue in seaborn 0.1…

…3.0 (#3569)

* Update _base.py

Corresponds to the case where grouping_var has None object.

* Add a test case

Add a test case to test_relational.py and a fixture empty dataframe.

* Fix error

To avoid the error 'The truth value of an array with more than one element is ambiguous.', we don't use walrus operator.

* Incorporate feedbacks.

- Remove unnecessary fixture.
- Refactoring.

seaborn/_base.py

@@ -933,7 +933,8 @@ def iter_data(
 
             grouping_keys = []
             for var in grouping_vars:
-                grouping_keys.append(levels.get(var, []))
+                key = levels.get(var)
+                grouping_keys.append([] if key is None else key)
 
             iter_keys = itertools.product(*grouping_keys)
             if reverse:

tests/test_relational.py

@@ -1302,6 +1302,9 @@ def test_lineplot_smoke(
         lineplot(x="x", y="y", hue="f", size="s", data=object_df)
         ax.clear()
 
+        lineplot(x="x", y="y", hue="a", data=long_df.iloc[:0])
+        ax.clear()
+
     def test_ci_deprecation(self, long_df):
 
         axs = plt.figure().subplots(2)