Fix wording in PrincipalExtensions.cs summary (#17859)

Add missing "if" in the summary.

src/Identity/Extensions.Core/src/PrincipalExtensions.cs

@@ -12,7 +12,7 @@ namespace System.Security.Claims
     public static class PrincipalExtensions
     {
         /// <summary>
-        /// Returns the value for the first claim of the specified type otherwise null the claim is not present.
+        /// Returns the value for the first claim of the specified type, otherwise null if the claim is not present.
         /// </summary>
         /// <param name="principal">The <see cref="ClaimsPrincipal"/> instance this method extends.</param>
         /// <param name="claimType">The claim type whose first value should be returned.</param>
@@ -28,4 +28,4 @@ namespace System.Security.Claims
         }
 
     }
-}
+}