Check the result of std::getenv() before constructing a std::string.
std::string cannot be constructed with nullptr.
Fix ambiguous overload resolution of std::tolower(), by wrapping it in a
lambda that explicitly takes a 'char' argument.
Bug: dawn:1123
Change-Id: I8a44524a6c0e71f7d2a49a5a9f730115c32df577
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/65960
Reviewed-by: Antonio Maiorano <amaiorano@google.com>
Commit-Queue: Ben Clayton <bclayton@google.com>