[Bug #20068] Encoding does not matter to empty strings

string.c

@@ -3586,7 +3586,7 @@ rb_str_prepend_multi(int argc, VALUE *argv, VALUE str)
 st_index_t
 rb_str_hash(VALUE str)
 {
-    int e = ENCODING_GET(str);
+    int e = RSTRING_LEN(str) ? ENCODING_GET(str) : 0;
     if (e && is_ascii_string(str)) {
         e = 0;
     }

test/ruby/test_string.rb

@@ -1344,6 +1344,7 @@ CODE
     assert_not_equal(S("a").hash, S("a\0").hash, bug4104)
     bug9172 = '[ruby-core:58658] [Bug #9172]'
     assert_not_equal(S("sub-setter").hash, S("discover").hash, bug9172)
+    assert_equal(S("").hash, S("".encode(Encoding::UTF_32BE)).hash)
   end
 
   def test_hex