Rollup merge of #92715 - chordtoll:empty-string, r=davidtwco

Do not suggest char literal for zero-length strings

PR #92507 adds a hint to switch to single quotes when a char is expected and a single-character string literal is provided.

The check to ensure the string literal is one character long missed the 0-char case, and would incorrectly offer the hint.

This PR adds the missing check, and a test case to confirm the new behavior.

compiler/rustc_infer/src/infer/error_reporting/mod.rs

@@ -2053,7 +2053,7 @@ impl<'a, 'tcx> InferCtxt<'a, 'tcx> {
                                 if let Some(code) =
                                     code.strip_prefix('"').and_then(|s| s.strip_suffix('"'))
                                 {
-                                    if code.chars().nth(1).is_none() {
+                                    if code.chars().count() == 1 {
                                         err.span_suggestion(
                                             span,
                                             "if you meant to write a `char` literal, use single quotes",