fix: length0 array repr

src/awkward/highlevel.py

@@ -1504,7 +1504,7 @@ def _repr_mimebundle_(self, include=None, exclude=None):
         # but we sort it from longest to shortest for _repr_mimebundle_
         sorted_rows = sorted(rows, key=lambda x: -len(x.split(":")[0]))
 
-        n_cols = max(map(len, header_lines))
+        n_cols = max(map(len, header_lines), default=0)
         body_lines = header_lines
         body_lines.append("-" * n_cols)
         body_lines.extend(sorted_rows)
@@ -2446,7 +2446,7 @@ def _repr_mimebundle_(self, include=None, exclude=None):
         # but we sort it from longest to shortest for _repr_mimebundle_
         sorted_rows = sorted(rows, key=lambda x: -len(x.split(":")[0]))
 
-        n_cols = max(map(len, header_lines))
+        n_cols = max(map(len, header_lines), default=0)
         body_lines = header_lines
         body_lines.append("-" * n_cols)
         body_lines.extend(sorted_rows)

tests/test_3564_length0_highlevel_repr.py

@@ -0,0 +1,12 @@
+from __future__ import annotations
+
+import awkward as ak
+
+
+def test():
+    length0 = ak.Array([])
+    assert repr(length0) == "<Array [] type='0 * unknown'>"
+    assert length0._repr_mimebundle_() == {
+        "text/html": "<pre>\nbackend: cpu\nnbytes: 0 B\ntype: 0 * unknown</pre>",
+        "text/plain": "<Array [] type='0 * unknown'>",
+    }