Fix lower size bounds in RandomBlobContent*Tests (#69771)

The repository analyzer does not write empty blobs to the repository,
and we assert that the blobs are nonempty, but the tests randomly check
for the empty case anyway. This commit ensures that the blobs used in
tests are always nonempty.

Relates #67247

Author: DaveCTurner

x-pack/plugin/snapshot-repo-test-kit/src/test/java/org/elasticsearch/repositories/blobstore/testkit/RandomBlobContentBytesReferenceTests.java

@@ -46,7 +46,7 @@ public void testStreamInput() throws IOException {
     }
 
     private static int randomSize() {
-        return between(0, 30000);
+        return between(1, 30000);
     }
 
 }

x-pack/plugin/snapshot-repo-test-kit/src/test/java/org/elasticsearch/repositories/blobstore/testkit/RandomBlobContentStreamTests.java

@@ -130,7 +130,7 @@ public void testReadingBytesThrowsExceptionAfterCancellation() {
     }
 
     private static int randomSize() {
-        return between(0, 30000);
+        return between(1, 30000);
     }
 
 }