Early out in load_buffer for empty inputs to avoid allocations

[zeux]

Previously, calling xml_document::load_string on an empty buffer resulted in a single byte allocation for the buffer copy; this allocation was redundant as no data was parsed, so we now add the same early-out that already exists in xml_parser::parse to load_buffer_impl.

This should almost never matter but might help applications that for some strange reason frequently parse empty documents without a higher level size check on the app side.