env: ensure we don't return empty PATH vars on Windows

Ensure that we don't return empty `PATH` values on Windows when
splitting. It's common that on Windows the `PATH` can be inadvertently
modified such that an empty value is included, and this can lead
to weird resolution of executables using the current directory.
As it stands, no existing codepaths would trigger this today when
invoked by Git for Windows, but let's make sure this never happens in
the future by mistake!

Author: mjcheetham

src/shared/Core/Interop/Windows/WindowsEnvironment.cs

@@ -22,7 +22,10 @@ internal WindowsEnvironment(IFileSystem fileSystem, IReadOnlyDictionary<string,
 
         protected override string[] SplitPathVariable(string value)
         {
-            return value.Split(';');
+            // Ensure we don't return empty values here - callers may use this as the base
+            // path for `Path.Combine(..)`, for which an empty value means 'current directory'.
+            // We only ever want to use the current directory for path resolution explicitly.
+            return value.Split(new[] { ';' }, StringSplitOptions.RemoveEmptyEntries);
         }
 
         public override void AddDirectoryToPath(string directoryPath, EnvironmentVariableTarget target)