Preserve empty SQLite strings

[antonsergeev88]

Motivation

• Ensure empty C strings returned by SQLite are preserved as empty Swift String values rather than being folded to nil.
• Make NULL and empty-string return values distinct in the Swift wrapper to avoid losing information from SQLite metadata and APIs.
• Align wrapper behavior with the intent of providing a thin, 1:1 mapping to SQLite semantics while keeping rawValue escape hatches.
• Address conversions used for statement SQL, column metadata, bind parameter names, database filenames, and result-code error strings.

Description

• Removed the guard !string.isEmpty checks that previously converted empty String to nil in Statement+SQL.swift, Statement+Column.swift, Statement+Bind.swift, Database+Filename.swift, and ResultCode.swift.
• Continued to use String(cString: ...) for C-to-Swift conversions and preserved the existing guard let cString nil checks to keep NULL distinct.
• No other API signatures or calling semantics were changed, and no new abstractions were introduced.
• Commit message: Preserve empty SQLite strings to reflect the behavior change.

Testing

• No automated tests were run for this change.
• Existing tests in the repository were not modified by this patch.
• Manual or CI validation was not requested in this rollout.
• Consider adding Swift Test cases that assert empty vs nil distinctions for the affected accessors in a follow-up.

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Codex Task