[7.1.0] Avoid using `InputStream.available()` to detect EOF while reading...

[7.1.0] Avoid using `InputStream.available()` to detect EOF while reading delimited protos. (#21143)

It's not a reliable method to check for EOF: it returns how many bytes
are guaranteed to be read without blocking, and the base implementation
always returns 0.

Instead, leverage the fact that parseDelimitedFrom() returns null if the
stream is at EOF.

PiperOrigin-RevId: 602257598
Change-Id: I61e51774611196fb44745dc0aa2dc836b41fcd68