Fix3355 - use default configuration with no build type

When configuring this extension with a CMakePresets.json, with a
CMakeCache.txt that doesn't define a CMAKE_BUILD_TYPE, the intelliSense
configuration was not passed to vscode-cpptools. However, this is a
valid CMake configuration which should use the toolchain's default
configuration. Building through the extension did work.

The codeModelContent configuration was rejected because it's name (""),
didn't match activeBuildTypeVariant which was null.

This patch accepts an unnamed configurations in the case where no active
build type variant is set.

src/cpptools.ts

@@ -593,7 +593,7 @@ export class CppConfigurationProvider implements cpptools.CustomConfigurationPro
         }
         for (const config of opts.codeModelContent.configurations) {
             // Update only the active build type variant.
-            if (config.name === opts.activeBuildTypeVariant) {
+            if (config.name === opts.activeBuildTypeVariant || (!opts.activeBuildTypeVariant && config.name === "")) {
                 for (const project of config.projects) {
                     for (const target of project.targets) {
                         // Now some shenanigans since header files don't have config data: