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

When px.histogram() is called with no data arguments, the histogram-
specific hover label path inserts a None key into mapping_labels
(because attr_label is None when no column is specified). The hover
template list comprehension then fails with:

    TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'

Skip adding to mapping_labels when attr_label is None, consistent
with how other chart types handle empty invocations.

Fixes #5534

Author: tysoncung

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"