[dotnet] Handle nullability on WebElement (#15225)

Author: RenderMichael

dotnet/src/webdriver/ILocatable.cs

@@ -20,6 +20,8 @@
 using OpenQA.Selenium.Interactions.Internal;
 using System.Drawing;
 
+#nullable enable
+
 namespace OpenQA.Selenium
 {
     /// <summary>

dotnet/src/webdriver/ITakesScreenshot.cs

@@ -17,6 +17,8 @@
 // under the License.
 // </copyright>
 
+#nullable enable
+
 namespace OpenQA.Selenium
 {
     /// <summary>