fix: handle empty px.histogram() by skipping None label in hover template

plotly/express/_core.py

@@ -588,7 +588,8 @@ def make_trace_kwargs(args, trace_spec, trace_data, mapping_labels, sizeref):
             and attr_name == "z"
         ):
             # ensure that stuff like "count" gets into the hoverlabel
-            mapping_labels[attr_label] = "%%{%s}" % attr_name
+            if attr_label is not None:
+                mapping_labels[attr_label] = "%%{%s}" % attr_name
     if trace_spec.constructor not in [go.Parcoords, go.Parcats]:
         # Modify mapping_labels according to hover_data keys
         # if hover_data is a dict

tests/test_optional/test_px/test_px_functions.py

@@ -619,3 +619,13 @@ def test_timeline_cols_already_temporal(constructor, datetime_columns):
     assert len(fig.data) == 3
     assert fig.layout.xaxis.type == "date"
     assert fig.layout.xaxis.title.text is None
+
+
+def test_empty_histogram():
+    """Empty px.histogram() should not raise, matching scatter/bar/pie behavior.
+
+    Regression test for https://github.com/plotly/plotly.py/issues/5534
+    """
+    fig = px.histogram()
+    assert len(fig.data) == 1
+    assert fig.data[0].type == "histogram"