Fix: bday conversion clobbers NICKNAME when FN is empty

The share-by-map BDAY-to-ICS conversion builds a placeholder mapping for
the SUMMARY/DESCRIPTION/alarm templates. Each block sets the fallback
marker for its own placeholder (e.g. {nickname} -> '!nickname!', {n:f} ->
'!n:f!'), but the {fn} block wrote the empty-FN fallback to {nickname}
instead of {fn}.

As a result, for a VCARD whose FN is present but empty, a genuine
NICKNAME was overwritten with '!fn!' in the generated event, and the
{fn} placeholder never received its '!fn!' marker, so a '[{fn}|...]'
fallback in a template failed to resolve.

Assign the fallback to {fn}, matching the surrounding blocks.

Co-Authored-By: Claude <noreply@anthropic.com>

radicale/item/__init__.py

@@ -717,7 +717,7 @@ class Item:
         if hasattr(self.vobject_item, "fn") and self.vobject_item.fn.value != "":
             placeholder_mapping['{fn}'] = self.vobject_item.fn.value
         else:
-            placeholder_mapping['{nickname}'] = '!fn!'
+            placeholder_mapping['{fn}'] = '!fn!'
 
         # rfc6350#6.2 FamilyName;GivenName;AdditionalNames;HonorificPrefixes;HonorificSuffixes
         if hasattr(self.vobject_item, "n") and self.vobject_item.n.value.family != "":

radicale/tests/test_sharing.py

@@ -28,11 +28,13 @@ import re
 import sys
 import tempfile
 import urllib
-from typing import Dict, Sequence, Tuple, Union
+from typing import Dict, Sequence, Tuple, Union, cast
 
 import pytest
+import vobject
 
-from radicale import pathutils, sharing, xmlutils
+from radicale import pathutils, sharing, storage, xmlutils
+from radicale.item import Item
 from radicale.tests import BaseTest
 from radicale.tests.helpers import get_file_content
 
@@ -5328,6 +5330,39 @@ permissions: RrWw""")
             assert "Test-FN-C3 (1990/5)" in answer
             assert "Test-FN-C3 (1990/6)" in answer
 
+    def test_sharing_bday_conversion_empty_fn(self) -> None:
+        """BDAY-to-ICS conversion of a VCARD with an empty FN property.
+
+        vCard mandates FN, but some clients emit it empty. The fallback for
+        an empty ``{fn}`` must be assigned to the ``{fn}`` placeholder, not to
+        ``{nickname}``: otherwise a present NICKNAME gets clobbered with the
+        ``!fn!`` marker and the ``{fn}`` fallback never triggers.
+        """
+        vcard = vobject.readOne(
+            "BEGIN:VCARD\r\n"
+            "VERSION:3.0\r\n"
+            "UID:contact-empty-fn\r\n"
+            "N:FamilyTest;GivenTest;;;\r\n"
+            "FN:\r\n"
+            "NICKNAME:Test-NICKNAME\r\n"
+            "BDAY:1990-05-06\r\n"
+            "END:VCARD\r\n")
+
+        class _StubCollection:
+            path = "test"
+
+        item = Item(collection=cast(storage.BaseCollection, _StubCollection()),
+                    vobject_item=vcard, href="contact-empty-fn.vcf")
+        converted = item.convert_vcf_to_ics(ShareActions={"config": {
+            "conversion_bday_summary_template": "{nickname} (BDAY)",
+            "conversion_bday_description_template": "[{fn}|no-fn]"}})
+        assert converted is not None
+        serialized = converted.serialize()
+        # NICKNAME must survive, not be overwritten by the '!fn!' marker
+        assert "SUMMARY:Test-NICKNAME (BDAY)" in serialized
+        # empty {fn} must expose the '!fn!' marker so the fallback resolves
+        assert "DESCRIPTION:no-fn" in serialized
+
     def test_sharing_api_map_vcf_bday_age_template(self) -> None:
         """share-by-map with conversion=bday template tests with age."""
         self.configure({"auth": {"type": "htpasswd",