[patch] Remove invalid test on non-const empty strings

[Paolo Carlini]

Paolo Carlini wrote:

> From a practical point of view, this idea goes in the right direction, in
> my opinion. Is __GLIBCXX_DEBUG_PEDANTIC meant to catch
> "undefined behavior" and "implemention defined behavior" usages?

I'm re-reading now our debug.html (sorry about the sloppiness) and indeed
it looks like the way to go is moving these debug-mode tests under
_GLIBCXX_DEBUG_PEDANTIC.

Paolo.