Fix classpath handling in EspressoLauncher

It should behave the same as the standard launcher.

Author: gilles-duboscq

espresso/src/com.oracle.truffle.espresso.launcher/src/com/oracle/truffle/espresso/launcher/EspressoLauncher.java

@@ -367,14 +367,12 @@ protected List<String> preprocessArguments(List<String> arguments, Map<String, S
             if (classpath == null) {
                 // (3) the environment variable CLASSPATH
                 classpath = System.getenv("CLASSPATH");
-                if (classpath == null) {
-                    // (4) the current working directory only
-                    classpath = ".";
-                }
             }
         }
 
-        espressoOptions.put("java.Classpath", classpath);
+        if (classpath != null) {
+            espressoOptions.put("java.Classpath", classpath);
+        }
 
         if (!isRelaxStaticObjectSafetyChecksSet) {
             // Since Espresso has a verifier, the Static Object Model does not need to perform shape