dawn_node: Fixes for GPU.cpp

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>

src/dawn_node/binding/GPU.cpp

@@ -31,7 +31,10 @@ namespace {
         }
         return "";
 #else
-        return std::getenv(varName);
+        if (auto* val = std::getenv(varName)) {
+            return val;
+        }
+        return "";
 #endif
     }
 }  // namespace
@@ -81,7 +84,8 @@ namespace wgpu { namespace binding {
 
         // Check for override from env var
         std::string envVar = getEnvVar("DAWNNODE_BACKEND");
-        std::transform(envVar.begin(), envVar.end(), envVar.begin(), std::tolower);
+        std::transform(envVar.begin(), envVar.end(), envVar.begin(),
+                       [](char c) { return std::tolower(c); });
         if (envVar == "null") {
             targetBackendType = wgpu::BackendType::Null;
         } else if (envVar == "webgpu") {