[SYCL] Coverity: avoid std::string_view(nullptr) in sycl-trace collectors

[crystarm]

Related to #18506.

Summary

Coverity reported potential null pointer issues in SYCL trace collectors where std::string_view was constructed directly from std::getenv(...).
If the environment variable is not set, this can result in std::string_view(nullptr) (UB).

Affected CIDs from the issue:
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"));

What changed

Replaced direct construction from std::getenv(...) with null-safe handling:

const char *PrinterTypeEnv = std::getenv("SYCL_TRACE_PRINT_FORMAT");
std::string_view PrinterType = PrinterTypeEnv ? PrinterTypeEnv : "";

Applied in:

• sycl/tools/sycl-trace/sycl_trace_collector.cpp
• sycl/tools/sycl-trace/ur_trace_collector.cpp
• sycl/tools/sycl-trace/verification_collector.cpp
• sycl/tools/sycl-trace/ze_trace_collector.cpp
• sycl/tools/sycl-trace/cuda_trace_collector.cpp (same pattern, proactively fixed)

Behavior impact

No functional change is intended for valid env values (classic, verbose, compact).
When SYCL_TRACE_PRINT_FORMAT is unset, behavior is now explicitly safe and well-defined.

[sys-ce-bb]

@intel/llvm-gatekeepers please consider merging