8034781: [TESTBUG] runtime/ClassFileUnsupportedClassFileVersion.java unable to...

8034781: [TESTBUG] runtime/ClassFileUnsupportedClassFileVersion.java unable to find or load main class
Summary: Passed empty string to ProcessBuilder which caused problems on Linux, fixed by checking string length
Reviewed-by: lfoltan, sla, mseledtsov

test/testlibrary/com/oracle/java/testlibrary/ProcessTools.java

@@ -154,7 +154,7 @@ public final class ProcessTools {
 
     if (addTestVmOptions) {
       String vmopts = System.getProperty("test.vm.opts");
-      if (vmopts != null) {
+      if (vmopts != null && vmopts.length() > 0) {
         Collections.addAll(args, vmopts.split("\\s"));
       }
     }