Fix empty string-view construction and assignment

[fhgffy]

Fixes #346.

A default-constructed string view has a null data pointer and zero length. Constructing or assigning a Boost.Container string from it currently aborts in assign(const CharT*, size_type):

boost::container::string s("previous value");
std::string_view empty;
s.assign(empty); // Assertion `s != 0` fails.

Accept a null pointer only when the count is zero and clear the destination before forming a pointer range. Positive-length and null-terminated inputs retain their existing non-null checks.

The regression covers Boost/std string views, char/wchar_t, default and non-null empty views, construction, assignment and pointer/count forwarding, empty/short/heap destinations, null termination, retained capacity, and later nonempty assignment.

Validation on GCC 14.2.0 / Linux:

• Regression fails at the assertion on unmodified develop 2d2f12280290f4d60096d1bfb521bb689adbe91d, passes with the fix
• Focused tests pass in C++03/11/14/17/20/23 with -Wall -Wextra -Werror -pedantic
• C++03 and C++17 no-exceptions configurations pass
• C++17 ASan/UBSan focused tests pass with assertions enabled and disabled; LeakSanitizer disabled because of the sandbox
• Full C++17 debug/release suite passes: 105 tests in each configuration; final incremental rerun also passes

Other compilers, standard libraries, architectures, and embedded devices were not tested locally. This change addresses construction and assignment; it does not change other string-view operations.

[igaztanaga]

Many thanks for your PR!