fix: concatenation of empty records

src/awkward/contents/recordarray.py

@@ -752,10 +752,13 @@ def _mergemany(self, others: Sequence[Content]) -> Content:
             ):
                 minlength = merged.length
 
-        if minlength is unknown_length:
+        # `not for_each_field`: is a corner-case when all
+        # the arrays are empty and have no fields either.
+        if minlength is unknown_length or not for_each_field:
+            from operator import attrgetter
+
             minlength = self.length
-            for x in others:
-                minlength += x.length
+            minlength += sum(map(attrgetter("length"), others))
 
         next = RecordArray(
             nextcontents,

tests/test_3258_concat_empty_records.py

@@ -0,0 +1,10 @@
+from __future__ import annotations
+
+import awkward as ak
+
+
+def test_concat_empty_records():
+    a = ak.Array([{}, {}, {}])
+    b = ak.Array([{}, {}])
+    c = ak.concatenate([a, b], axis=0)
+    assert c.to_list() == [{}, {}, {}, {}, {}]