patch 8.2.3048: strange error for white space after ++ command

Problem:    Strange error for white space after ++ command.
Solution:   Check for white space explicitly. (closes #8440)

src/testdir/test_vim9_assign.vim

@@ -1906,6 +1906,12 @@ def Test_inc_dec()
       unlet g:count
   END
   CheckDefAndScriptSuccess(lines)
+
+  lines =<< trim END
+      var nr = 7
+      ++ nr
+  END
+  CheckDefAndScriptFailure(lines, "E1202: No white space allowed after '++': ++ nr")
 enddef