Parse <??> as syntactically valid processing instruction

Although empty target name isnot allowed, we would check that (or change parsing)
the same way as currently we check the presence of mandatory `version` attribute
in XML declaration. This requires introducing separate type for PI content which
will be done in tafia#650

Fixed (3):
    syntax::pi::normal_empty::async_tokio
    syntax::pi::normal_empty::borrowed
    syntax::pi::normal_empty::buffered

failures (3):
    syntax::decl::normal1::async_tokio
    syntax::decl::normal1::borrowed
    syntax::decl::normal1::buffered

Changelog.md

@@ -42,6 +42,8 @@ configuration is serializable.
   - `Error::EmptyDocType` replaced by `IllFormedError::MissedDoctypeName` (in [#684])
 - [#684]: Changed positions reported for `SyntaxError`s: now they are always points
   to the start of markup (i. e. to the `<` character) with error.
+- [#684]: Now `<??>` parsed as `Event::PI` with empty content instead of raising
+  syntax error.
 
 [#513]: https://github.com/tafia/quick-xml/issues/513
 [#622]: https://github.com/tafia/quick-xml/issues/622

src/reader/state.rs

@@ -190,8 +190,13 @@ impl ReaderState {
     ///
     /// Returns `Decl` or `PI` event
     pub fn emit_question_mark<'b>(&mut self, buf: &'b [u8]) -> Result<Event<'b>> {
+        debug_assert!(buf.len() > 0);
+        debug_assert_eq!(buf[0], b'?');
+
         let len = buf.len();
-        if len > 2 && buf[len - 1] == b'?' {
+        // We accept at least <??>
+        //                     ~~ - len = 2
+        if len > 1 && buf[len - 1] == b'?' {
             if len > 5 && &buf[1..4] == b"xml" && is_whitespace(buf[4]) {
                 let event = BytesDecl::from_start(BytesStart::wrap(&buf[1..len - 1], 3));