Fixed lexical error when a C++ file contains an empty character literal L''.

This commit fixes a lexical error when a C++ file contains an empty
character literal L''. The following code could not be tokenized:

std::wstring wsMessage( sMessage.length(), L'');

It triggers the following error:

net.sourceforge.pmd.lang.ast.TokenMgrError: Lexical error in file
'foo.cpp' at line 1, column 46.  Encountered: "\'" (39), after : "\'"

pmd-cpp/etc/grammar/cpp.jj

@@ -319,7 +319,7 @@ TOKEN :
 {
 
   <  CHARACTER : ("L")? "'"
-   (   (~["'","\\","\n","\r"])+
+   (   (~["'","\\","\n","\r"])*
    | ("\\" (
              ["n","t","v","b","r","f","a","\\","?","'","\""]
             |

pmd-cpp/src/test/java/net/sourceforge/pmd/cpd/CPPTokenizerTest.java

@@ -107,6 +107,12 @@ public class CPPTokenizerTest {
         parse("#define LSTFVLES_CPP  //*" + PMD.EOL);
     }
 
+    @Test
+    public void testEmptyCharacter() {
+        Tokens tokens = parse("std::wstring wsMessage( sMessage.length(), L'');" + PMD.EOL);
+        assertEquals(15, tokens.size());
+    }
+
     private Tokens parse(String snippet) {
         return parse(snippet, false);
     }