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>

CHANGELOG.md

@@ -4,6 +4,7 @@
 * Fix: time-range filter treated a VEVENT with a whole-day DURATION (e.g. P1D, P2D) as zero-length (timedelta.seconds instead of total_seconds), so such events were missing from calendar-query REPORT results
 * Fix: calendar-data expand (REPORT) left recurrence properties (e.g. RDATE) on the expanded single-occurrence VEVENTs; a single try/except around the sequential delattr() calls stopped at the first absent property (e.g. missing EXDATE), so later ones were never removed
 * Fix: text-match filter on a structured property (e.g. vCard N or ADR) crashed with HTTP 500 (AttributeError: 'Name'/'Address' object has no attribute 'lower') because vobject parses these into non-string objects; their text representation is now used
+* Fix: sharing bday-to-ICS conversion assigned the empty-FN fallback marker to the {nickname} placeholder instead of {fn}, so a VCARD with an empty FN got its NICKNAME overwritten with "!fn!" in the generated SUMMARY/DESCRIPTION and the {fn} fallback never resolved
 
 ## 3.7.6
 * Extension: item verification on commandline