Change exception throwing in Path.GetRelativePath (dotnet/corefx#38388)

* Fix Path.GetRelativePath throws ArgumentNullException when relativeTo is empty string

* Code review fixes


Commit migrated from dotnet/corefx@249b512

src/libraries/System.Runtime.Extensions/tests/System/IO/Path.GetRelativePath.cs

@@ -148,5 +148,12 @@ public static void GetRelativePath_Mac(string relativeTo, string path, string ex
                 ignoreLineEndingDifferences: false,
                 ignoreWhiteSpaceDifferences: false);
         }
+
+        [Fact]
+        public static void GetRelativePath_InvalidArgs()
+        {
+            AssertExtensions.Throws<ArgumentNullException>("relativeTo", null, () => Path.GetRelativePath(null, "."));
+            AssertExtensions.Throws<ArgumentException>("relativeTo", null, () => Path.GetRelativePath(string.Empty, "."));
+        }
     }
 }