libbinder_ndk: toString never goes to empty strings.

This needs more follow-up, but we should show values directly.

Before, an array of a single valid PFD would look like an empty array
"[]". Now, we distinguish an empty array "[]" from "[fd:3]".

Fixes: 260177075
Change-Id: I7c8ea833e999c8cda24db92fa1ac84b86a51602f
Test: N/A