[refurb] Detect empty f-strings (FURB105)

[danparizher]

Summary

Fixes FURB105 (print-empty-string) to detect empty f-strings in addition to regular empty strings. Previously, the rule only flagged print("") but missed print(f""). This fix ensures both cases are detected and can be automatically fixed.

Fixes #21346

Problem Analysis

The FURB105 rule checks for unnecessary empty strings passed to print() calls. The is_empty_string helper function was only checking for Expr::StringLiteral with empty values, but did not handle Expr::FString (f-strings). As a result, print(f"") was not being flagged as a violation, even though it's semantically equivalent to print("") and should be simplified to print().

The issue occurred because the function used a matches! macro that only checked for string literals:

fn is_empty_string(expr: &Expr) -> bool {
    matches!(
        expr,
        Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) if value.is_empty()
    )
}

Approach

1. Import the helper function: Added is_empty_f_string to the imports from ruff_python_ast::helpers, which already provides logic to detect empty f-strings.

2. Update is_empty_string function: Changed the implementation from a matches! macro to a match expression that handles both string literals and f-strings:

   fn is_empty_string(expr: &Expr) -> bool {
       match expr {
           Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) => value.is_empty(),
           Expr::FString(f_string) => is_empty_f_string(f_string),
           _ => false,
       }
   }

The fix leverages the existing is_empty_f_string helper function which properly handles the complexity of f-strings, including nested f-strings and interpolated expressions. This ensures the detection is accurate and consistent with how empty strings are detected elsewhere in the codebase.

[ntBre]

Thank you!

I'm just going to close and reopen to try to trigger the ecosystem comment.

[ntBre]

Still no comment, but I downloaded the artifact:

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