fix: guard constructTweet against undefined full_text

The longShortClass calculation reads t.full_text.length without a
guard, throwing a TypeError when a tweet object has no full_text
property (observed in v1.9.6.5 during renderTimeline on profile
pages). Fall back to length 0 when full_text is missing so the
class assignment matches the existing short-tweet behaviour.

Author: SAY-5

scripts/tweetConstructor.js

@@ -669,7 +669,7 @@ async function constructTweet(t, tweetConstructorArgs, options = {}) {
     // Main text content
     const longShortClass =
         vars.noBigFont ||
-        t.full_text.length > 280 ||
+        (t.full_text?.length ?? 0) > 280 ||
         !options.bigFont ||
         (!options.mainTweet && location.pathname.includes("/status/"))
             ? "tweet-body-text-long"