[Coverity] Potential `nullptr` dereference in various sycl tools

[AlexeySachkov]

See Coverity issues at https://scan.coverity.com/projects/intel-llvm?tab=analysis_settings

CID 437235:

llvm/sycl/tools/sycl-trace/sycl_trace_collector.cpp

Line 138 in f9a7cc8

std::string_view PrinterType(std::getenv("SYCL_TRACE_PRINT_FORMAT"));

CID 442884:

llvm/sycl/tools/sycl-trace/ur_trace_collector.cpp

Line 91 in f9a7cc8

std::string_view PrinterType(std::getenv("SYCL_TRACE_PRINT_FORMAT"));

CID 440717:

llvm/sycl/tools/sycl-trace/verification_collector.cpp

Line 27 in f9a7cc8

std::string_view PrinterType(std::getenv("SYCL_TRACE_PRINT_FORMAT"));

CID 437184:

llvm/sycl/tools/sycl-trace/ze_trace_collector.cpp

Line 311 in f9a7cc8

std::string_view PrinterType(std::getenv("SYCL_TRACE_PRINT_FORMAT"));

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The problem here is that string_view(nullptr) behavior is not defined.
string_veiw(const chatT *str) constructor has a precondition that [str, str + char_traits<charT>::length(str)) is a valid range (string.view.cons.2).

length is defined by char.traits.require as:

Returns: the smallest i such that X​::​eq(p[i],charT()) is true.

When there is no such i when p is nullptr. See also p0903r0

string_view(nullptr) constructor is explicitly deleted in C++23: cppreference.