Fix allowing empty string to be passed to resources

[SteveL-MSFT]

PR Summary

The existing code always tries to parse a string as an expression and handles specific cases where it fails to have it as a literal. In the case like an empty string, the parser fails immediately so there's no chance to pass an empty string.

The fix checks if the trimmed statement is empty and if so, passes it back as a string literal instead of trying to parse.

PR Context

Fix #1542

[Copilot]

Pull request overview

This PR updates the DSC expression parser to allow empty-string property values to flow through expression evaluation without failing parsing, addressing #1542.

Changes:

• Treat a tree-sitter root parse error as a literal string value instead of a parser error.
• Add a Pester test validating that an empty string property value is preserved through dsc config get.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
lib/dsc-lib/src/parser/mod.rs	Adjusts root-node parse error handling so empty strings can be passed as literal values.
dsc/tests/dsc_expressions.tests.ps1	Adds coverage for empty-string property values during expression evaluation.