livecheck: refactor HEAD-only formula handling

The existing code for handling a `HEAD`-only formula involves two
return values that can be `nil` but this isn't apparent because the
related methods aren't typed. This adds type signatures to the
methods and updates the livecheck code to account for `nil` return
values (making it clear which methods can return `nil`).

Co-authored-by: Mike McQuaid <mike@mikemcquaid.com>

Library/Homebrew/cask/dsl.rb

@@ -287,6 +287,7 @@ module Cask
     #
     # @see DSL::Version
     # @api public
+    sig { params(arg: T.nilable(T.any(String, Symbol))).returns(T.nilable(DSL::Version)) }
     def version(arg = nil)
       set_unique_stanza(:version, arg.nil?) do
         if !arg.is_a?(String) && arg != :latest

Library/Homebrew/formula.rb

@@ -2403,6 +2403,7 @@ class Formula
 
   # Returns the {PkgVersion} for this formula if it is installed.
   # If not, return `nil`.
+  sig { returns(T.nilable(PkgVersion)) }
   def any_installed_version
     any_installed_keg&.version
   end

Library/Homebrew/keg.rb

@@ -388,6 +388,7 @@ class Keg
     (path/"share/emacs/site-lisp"/name).children.any? { |f| ELISP_EXTENSIONS.include? f.extname }
   end
 
+  sig { returns(PkgVersion) }
   def version
     require "pkg_version"
     PkgVersion.parse(path.basename.to_s)

Library/Homebrew/livecheck/livecheck.rb

@@ -13,6 +13,8 @@ module Homebrew
   # command. These methods print the requested livecheck information
   # for formulae.
   module Livecheck
+    NO_CURRENT_VERSION_MSG = "Unable to identify current version"
+
     UNSTABLE_VERSION_KEYWORDS = T.let(%w[
       alpha
       beta
@@ -249,13 +251,20 @@ module Homebrew
         # comparison.
         current = if formula
           if formula.head_only?
-            Version.new(formula.any_installed_version.version.commit)
-          else
-            T.must(formula.stable).version
+            formula_commit = formula.any_installed_version&.version&.commit
+            Version.new(formula_commit) if formula_commit
+          elsif (stable = formula.stable)
+            stable.version
           end
         else
           Version.new(formula_or_cask.version)
         end
+        unless current
+          raise Livecheck::Error, NO_CURRENT_VERSION_MSG unless json
+          next if quiet
+
+          next status_hash(formula_or_cask, "error", [NO_CURRENT_VERSION_MSG], full_name: use_full_name, verbose:)
+        end
 
         current_str = current.to_s
         current = LivecheckVersion.create(formula_or_cask, current)