Rollup merge of #139391 - TaKO8Ki:check-if-merged-attrs-list-is-empty, r=jdonszelmann

Check if merged attributes list is empty in expr

Fixes #139373

In the example code, an [`UnrecognizedReprHint`](https://github.com/rust-lang/rust/blob/6b5ccfc87f59ab017032e430d4d358b4989735da/compiler/rustc_attr_parsing/src/attributes/repr.rs#L155) error is output, and the list of merged attributes becomes empty. This causes a [panic](https://github.com/rust-lang/rust/blob/6b5ccfc87f59ab017032e430d4d358b4989735da/compiler/rustc_ast_lowering/src/lib.rs#L618) to occur. So, it's necessary to check if merged attributes list is empty as other functions do.

ref: https://github.com/rust-lang/rust/blob/6b5ccfc87f59ab017032e430d4d358b4989735da/compiler/rustc_ast_lowering/src/lib.rs#L896

Author: GuillaumeGomez