Fixup test

Author: vchuravy

test/ccall.jl

@@ -1974,7 +1974,7 @@ end
 
 let llvm = sprint(code_llvm, gc_safe_ccall, ())
     # check that the call works
-    @test gc_safe_ccall() isa String
+    @test gc_safe_ccall() isa Nothing
     # check for the gc_safe store
     @test occursin("store atomic i8 2", llvm)
 end