Do not produce empty IDs in simple versioning test

Empty IDs are rejected during indexing, so we should not randomly
produce them during tests. This commit modifies the simple versioning
tests to no longer produce empty IDs.

Author: jasontedor

core/src/test/java/org/elasticsearch/versioning/SimpleVersioningIT.java

@@ -358,7 +358,7 @@ private IDSource getRandomIDs() {
                 ids = new IDSource() {
                     @Override
                     public String next() {
-                        return TestUtil.randomSimpleString(random);
+                        return TestUtil.randomSimpleString(random, 1, 10);
                     }
                 };
                 break;
@@ -368,7 +368,7 @@ public String next() {
                 ids = new IDSource() {
                     @Override
                     public String next() {
-                        return TestUtil.randomRealisticUnicodeString(random);
+                        return TestUtil.randomRealisticUnicodeString(random, 1, 20);
                     }
                 };
                 break;