Rollup merge of #90657 - GuillaumeGomez:one-char-last-line-removed, r=jyn514

Fix bug with `#[doc]` string single-character last lines

Fixes #90618.

This is because `.iter().all(|c| c == '*')` returns `true` if there is no character checked. And in case the last line has only one character, it simply returns `true`, making the last line behind removed.

src/test/rustdoc/include_str_cut.rs

@@ -0,0 +1,7 @@
+#![crate_name = "foo"]
+#![no_std]
+
+// @has 'foo/fn.foo.html'
+// @has - '//*[@class="docblock"]' 'inc2 x'
+#[doc = include_str!("short-line.md")]
+pub fn foo() {}

src/test/rustdoc/short-line.md

@@ -0,0 +1,2 @@
+inc2
+x