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

[Paolo Carlini]

Jonathan Wakely wrote:

>There's already __GLIBCXX_DEBUG_PEDANTIC
>
>Maybe the non-pedantic debug mode should be changed to allow this v3
>extension, but refuse it in pedantic mode.
>  
>
 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?

Paolo.