[Iguana] Display empty stack items as ""

Summary:
Currently, empty stack items are simply displayed as the empty string, however, in practice this is somewhat confusing.

Instead, we display "", which is easy to understand and also consistent with how the empty string is displayed on the explorer.

Note that the explorer also displays hex strings prefixed with "0x", which might be appropriate for stack items for iguana, too.

In the future, we could check if the encoded bytes are all ASCII (including \0) and display the decoded ASCII in quotes.

Test Plan: ninja check-iguana

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D16982