Improves `SorobanDataBuilder` construction and adds a getter

[Shaptic]

This does two things:

• change: the default structure will kick in on any "falsy" parameter, now (before, it considered string/buffer first, but this meant empty strings, e.g. from Soroban RPC, were interpreted incorrectly)
• add: getFootprint() to easily get both read-only and read-write pieces

[github-actions[bot]]

Size Change: -2.72 kB (0%)

Total Size: 3.08 MB

Filename	Size	Change
dist/stellar-base.js	2.26 MB	-2.91 kB (0%)
dist/stellar-base.min.js	821 kB	+185 B (0%)

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