seq, java: fix allocation bug in empty strings and test

Fixes golang/go#9271.

Change-Id: I57dcd11ae5afdc23bc9b1a0945d0789c9feeefb1
Reviewed-on: https://go-review.googlesource.com/1423
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>

bind/java/SeqTest.java

@@ -38,6 +38,20 @@ public class SeqTest extends TestCase {
     assertEquals("Strings should match", want, got);
   }
 
+  public void testNilErr() throws Exception {
+    Testpkg.Err(null); // returns nil, no exception
+  }
+
+  public void testErr() {
+    String msg = "Go errors are dropped into the confusing space of exceptions";
+    try {
+      Testpkg.Err(msg);
+      fail("expected non-nil error to be turned into an exception");
+    } catch (Exception e) {
+      assertEquals("messages should match", msg, e.getMessage());
+    }
+  }
+
   public void testGoRefGC() {
     Testpkg.S s = Testpkg.New();
     runGC();