Empty code block in doc causes UCM exception

Describe and demonstrate the bug

Input in scratch.u:

README = {{{

}}

Then save the editor:

Encountered exception:
Prelude.last: empty list
CallStack (from HasCallStack):
  error, called at libraries/base/GHC/List.hs:1644:3 in base:GHC.List
  errorEmptyList, called at libraries/base/GHC/List.hs:158:13 in base:GHC.List
  lastError, called at libraries/base/GHC/List.hs:153:29 in base:GHC.List
  last, called at src/Unison/Syntax/Lexer/Unison.hs:432:23 in unison-syntax-0.0.0-45p2Y49CWcnC6rok1OkVIc:Unison.Syntax.Lexer.Unison

Environment (please complete the following information):

unison version: release/0.5.26 (built on 2024-09-05)
MacOS 14.6.1 on Macbook Pro M1 Max

unisonweb / unison

Empty code block in doc causes UCM exception #5349