fix: give an empty request body the shape express gives it

[nigrosimone]

A request with content-length: 0 returned early before any parser ran, leaving req.body as the placeholder object every parser starts from. Express runs the parser regardless, and each one produces its own empty value.

app.post('/upload', express.raw(), (req, res) => {
    hash.update(req.body);   // Buffer on express, plain object here -> throws
});

Same for express.text(), where express hands back '' and this handed back an object.

parser	express 4	express 5	uExpress before	uExpress after
json	{}	{}	{}	{}
urlencoded	{}	{}	{}	{}
text	''	''	{}	''
raw	empty Buffer	empty Buffer	{}	empty Buffer

json and urlencoded happened to line up because the placeholder is object-shaped; text and raw did not.

What changed

The empty case now runs the parser tail directly with an empty buffer instead of returning early, and it does so after the content-type checks, so a parser still ignores a request it does not handle. Nothing is read, so the cost is one zero-length buffer allocation on a path that previously did nothing.

All four tails already handled an empty buffer — json even has an explicit if(buf.length === 0) branch — so each one produces what it should without further changes.

One thing deliberately left alone

When a parser does not claim a request, express 4 leaves req.body as {} and express 5 leaves it undefined. uExpress follows express 4, which is what the suite enforces. That divergence predates this change and is not touched here — the test avoids that case so it can pass cleanly against both express versions.