Handle chat messages without sender GUIDs

[Minnona]

Summary

• ignore absent, empty, or malformed sender GUIDs when resolving chat class colors
• share one protected GUID lookup between live chat, name caching, and GUID caching
• avoid building name-with-realm cache entries when player metadata is incomplete

Root cause

The class-color path checked arg12 ~= "", which is also true when arg12 is nil. Ascension chat events can omit Blizzard's optional sender GUID, causing GetPlayerInfoByGUID(nil) to raise a usage error. The later player-name cache protected the API call with pcall, but could still pass its nil realm result to strmatch.

Impact

Chat messages without sender GUID metadata render normally without Lua errors. Messages with valid GUIDs retain their existing class-color and name-cache behavior.

Validation

• luac5.1 -p ElvUI/Modules/Chat/Chat.lua
• git diff --check upstream/main...HEAD
• focused helper checks for nil, empty, valid, and malformed GUID inputs
• installed as part of the combined local build
• exercised under normal in-game chat activity after reload with no recurrence or other Lua errors