[JVM] Default to empty string in nqp::execname

Otherwise we'd need an additional null check for the JVM backend,
while on other backends, we're just checking if nqp::execname()
gave an empty string.

src/vm/jvm/runtime/org/raku/nqp/runtime/Ops.java

@@ -5516,7 +5516,7 @@ else if (env.containsKey("perl6.execname"))
         else if (env.containsKey("nqp.execname"))
             return env.getProperty("nqp.execname");
 
-        return null;
+        return "";
     }
 
     /* Thread related. */