[`ruff`] Use helper function for empty f-string detection in `in-empty-collection` (`RUF060`) (#20249)

## Summary

Fixes #20238

Replace inline f-string emptiness check with `is_empty_f_string` helper
function

crates/ruff_linter/resources/test/fixtures/ruff/RUF060.py

@@ -42,3 +42,7 @@ b"a" in bytes("a", "utf-8")
 1 in set(set([1]))
 '' in {""}
 frozenset() in {frozenset()}
+
+# https://github.com/astral-sh/ruff/issues/20238
+"b" in f"" "" # Error
+"b" in f"" "x" # OK

crates/ruff_linter/src/rules/ruff/rules/in_empty_collection.rs

@@ -1,5 +1,5 @@
 use ruff_macros::{ViolationMetadata, derive_message_formats};
-use ruff_python_ast::{self as ast, CmpOp, Expr};
+use ruff_python_ast::{self as ast, CmpOp, Expr, helpers::is_empty_f_string};
 use ruff_python_semantic::SemanticModel;
 use ruff_text_size::Ranged;
 
@@ -75,10 +75,7 @@ fn is_empty(expr: &Expr, semantic: &SemanticModel) -> bool {
         Expr::Dict(ast::ExprDict { items, .. }) => items.is_empty(),
         Expr::BytesLiteral(ast::ExprBytesLiteral { value, .. }) => value.is_empty(),
         Expr::StringLiteral(ast::ExprStringLiteral { value, .. }) => value.is_empty(),
-        Expr::FString(s) => s
-            .value
-            .elements()
-            .all(|elt| elt.as_literal().is_some_and(|elt| elt.is_empty())),
+        Expr::FString(s) => is_empty_f_string(s),
         Expr::Call(ast::ExprCall {
             func,
             arguments,

crates/ruff_linter/src/rules/ruff/snapshots/ruff_linter__rules__ruff__tests__RUF060_RUF060.py.snap

@@ -251,3 +251,12 @@ RUF060 Unnecessary membership test on empty collection
 25 |
 26 | # OK
    |
+
+RUF060 Unnecessary membership test on empty collection
+  --> RUF060.py:47:1
+   |
+46 | # https://github.com/astral-sh/ruff/issues/20238
+47 | "b" in f"" "" # Error
+   | ^^^^^^^^^^^^^
+48 | "b" in f"" "x" # OK
+   |