Check for null Path (#4391)

Author: lewing

src/Microsoft.TestPlatform.TestHostProvider/Hosting/DotnetTestHostManager.cs

@@ -777,7 +777,10 @@ private string GetTestHostPath(string runtimeConfigDevPath, string depsFilePath,
                             }
                         }
 
-                        testHostPath = Path.Combine(testhostPackage.Path, testHostPath);
+                        if (testhostPackage.Path is not null)
+                        {
+                            testHostPath = Path.Combine(testhostPackage.Path, testHostPath);
+                        }
                         _hostPackageVersion = testhostPackage.Version;
                         IsVersionCheckRequired = !_hostPackageVersion.StartsWith("15.0.0");
                         EqtTrace.Verbose("DotnetTestHostmanager: Relative path of testhost.dll with respect to package folder is {0}", testHostPath);