[Truffle] Fixed some more String#{chomp, chomp\!} cases.

Author: nirvdrum

spec/truffle/tags/core/string/chomp_tags.txt

@@ -1,9 +1,6 @@
 fails:String#chomp when passed no argument removes one trailing carrige return, newline pair
 fails:String#chomp when passed no argument taints the result if self is tainted
-fails:String#chomp when passed nil does not modify the String
-fails:String#chomp when passed nil returns a copy of the String
 fails:String#chomp when passed nil taints the result if self is tainted
-fails:String#chomp when passed nil returns an empty String when self is empty
 fails:String#chomp when passed '' removes a final newline
 fails:String#chomp when passed '' removes a final carriage return, newline
 fails:String#chomp when passed '' removes more than one trailing newlines
@@ -18,10 +15,7 @@ fails:String#chomp! raises a RuntimeError on a frozen instance when it would not
 fails:String#chomp! when passed no argument returns nil if self is not modified
 fails:String#chomp! when passed no argument removes one trailing newline
 fails:String#chomp! when passed no argument removes one trailing carrige return, newline pair
-fails:String#chomp! when passed no argument returns nil when self is empty
 fails:String#chomp! when passed no argument removes trailing characters that match $/ when it has been assigned a value
-fails:String#chomp! when passed nil returns nil
-fails:String#chomp! when passed nil returns nil when self is empty
 fails:String#chomp! when passed '' removes a final newline
 fails:String#chomp! when passed '' removes a final carriage return, newline
 fails:String#chomp! when passed '' does not remove a final carriage return

truffle/src/main/java/org/jruby/truffle/nodes/core/StringNodes.java

@@ -512,14 +512,23 @@ public ChompBangNode(ChompBangNode prev) {
         }
 
         @Specialization
-        public RubyString chompBang(RubyString string, UndefinedPlaceholder undefined) {
+        public Object chompBang(RubyString string, UndefinedPlaceholder undefined) {
             notDesignedForCompilation();
 
+            if (string.length() == 0) {
+                return getContext().getCoreLibrary().getNilObject();
+            }
+
             string.set(StringNodesHelper.chomp(string));
             return string;
         }
 
-        @Specialization(guards = "!isUndefinedPlaceholder(arguments[1])")
+        @Specialization
+        public RubyNilClass chompBangWithNil(RubyString string, RubyNilClass stringToChomp) {
+            return getContext().getCoreLibrary().getNilObject();
+        }
+
+        @Specialization(guards = { "!isUndefinedPlaceholder(arguments[1])", "!isRubyNilClass(arguments[1])" })
         public RubyString chompBangWithString(VirtualFrame frame, RubyString string, Object stringToChomp) {
             notDesignedForCompilation();