internal/importers: always specify classpath to javap

To avoid picking up a classpath from the CLASSPATH environment
variable, always specify an explicit classpath to javap, even if is
empty.

Updates golang/go#19046.

Change-Id: I2b3d941ba2779f90654117a046ae1600a53d5848
Reviewed-on: https://go-review.googlesource.com/37320
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
diff --git a/internal/importers/java/java.go b/internal/importers/java/java.go
index b9bf3bd..b294e6d 100644
--- a/internal/importers/java/java.go
+++ b/internal/importers/java/java.go
@@ -598,9 +598,7 @@
 		return nil, nil
 	}
 	args := []string{"-J-Duser.language=en", "-s", "-protected", "-constants"}
-	if j.Classpath != "" {
-		args = append(args, "-classpath", j.Classpath)
-	}
+	args = append(args, "-classpath", j.Classpath)
 	if j.Bootclasspath != "" {
 		args = append(args, "-bootclasspath", j.Bootclasspath)
 	}