fix: let form mappings apply a behavior (#834)

* allow a behavior to be fed from a form_mapping

* update _get_meta_array

src/uproot/_dask.py

@@ -781,8 +781,8 @@ def _get_meta_array(
     if form_mapping is not None:
         form = form_mapping(form)
 
-    empty_arr = awkward.from_buffers(
-        form, 0, {"": b"\x00\x00\x00\x00\x00\x00\x00\x00"}, buffer_key=""
+    empty_arr = form.length_zero_array(
+        behavior=None if form_mapping is None else form_mapping.behavior
     )
 
     return dask_awkward.core.typetracer_array(empty_arr), form
@@ -926,6 +926,7 @@ def foreach(start):
         ),
         partition_args,
         label="from-uproot",
+        behavior=None if form_mapping is None else form_mapping.behavior,
         meta=meta,
     )
 
@@ -980,5 +981,6 @@ def _get_dak_array_delay_open(
         ),
         files,
         label="from-uproot",
+        behavior=None if form_mapping is None else form_mapping.behavior,
         meta=meta,
     )