formula_installer: return nil from recursive_requirements block

The else branch ends with `unsatisfied_reqs[dependent] << req`,
which returns an Array. This violates the block's declared return
type of T.nilable(Symbol). Add an explicit nil return, matching
the same pattern used in language/python.rb.

Author: dduugg

Library/Homebrew/formula.rb

@@ -2632,9 +2632,9 @@ def conflicts = T.must(self.class.conflicts)
       block: T.nilable(
         T.proc.params(
           arg0: T.any(Formula, CaskDependent),
-          arg1: Dependency
-        ).returns(T.nilable(Symbol))
-      )
+          arg1: Dependency,
+        ).returns(T.nilable(Symbol)),
+      ),
     ).returns(T::Array[Dependency])
   }
   def recursive_dependencies(&block)
@@ -2651,14 +2651,15 @@ def recursive_dependencies(&block)
   # The full set of {Requirements} for this formula's dependency tree.
   #
   # @api internal
-  sig {
+  T::Sig::WithoutRuntime.sig {
+    # CaskDependent may not be initialized yet, so we don't use a runtime sig
     params(
       block: T.nilable(
         T.proc.params(
           arg0: T.any(Formula, CaskDependent, SoftwareSpec),
-          arg1: Requirement
-        ).returns(T.nilable(Symbol))
-      )
+          arg1: Requirement,
+        ).returns(T.nilable(Symbol)),
+      ),
     ).returns(Requirements)
   }
   def recursive_requirements(&block)

Library/Homebrew/formula_installer.rb

@@ -736,6 +736,7 @@ def expand_requirements
           next Dependable::PRUNE
         else
           unsatisfied_reqs[dependent] << req
+          nil # Return nil to satisfy T.nilable(Symbol) block sig (Array from << would violate it).
         end
       end
     end