Return EMPTY_US_ASCII for TStringConstants.lookupUSASCIITString("")

* CoreSymbols#NULL uses that, and relies on the TruffleString to be in
  the TStringCache to avoid duplicates.

Author: eregon

src/main/java/org/truffleruby/core/string/TStringConstants.java

@@ -220,7 +220,9 @@ private static TruffleString ascii(String string) {
     }
 
     public static TruffleString lookupUSASCIITString(String string) {
-        if (string.length() == 1) {
+        if (string.isEmpty()) {
+            return EMPTY_US_ASCII;
+        } else if (string.length() == 1) {
             return US_ASCII_SINGLE_BYTE[string.charAt(0)];
         } else {
             return TSTRING_CONSTANTS.get(string);