Fix ReadFromBuffer() for empty strings in debug mode

[meyerj]

If the string to be read is empty (string_size == 0) and the package that includes this header is compiled in debug mode, then this assertion in Span::operator[] can fail because buffer[offset] would access the span past its boundaries if this is the last element in the buffer.

I assume that the implementations of std::string::assign(buffer_ptr, string_size) ignore the buffer_ptr if string_size == 0, but better to handle this case separately...

Edit: Actually it is not an assertion and span.hpp defines its own macros for checking contracts and either throw exceptions or terminate, but the result is the same.

[facontidavide]

thanks for the fix.

I should port this to the parser in PlotJuggler, too.