Remove 0-value optimization from Add{Micro,Nano}Seconds

Returning early for 0 is only an optimization when passing in a 0.
I estimate that most usages do not that, as why would you call `AddMicroSeconds(0)`.

Src/FluentAssertions/Extensions/FluentDateTimeExtensions.cs

@@ -257,11 +257,6 @@ public static int Nanosecond(this DateTimeOffset self)
     /// </summary>
     public static DateTime AddNanoseconds(this DateTime self, long nanoseconds)
     {
-        if (nanoseconds == 0)
-        {
-            return self;
-        }
-
         return self + nanoseconds.Nanoseconds();
     }
 
@@ -270,11 +265,6 @@ public static DateTime AddNanoseconds(this DateTime self, long nanoseconds)
     /// </summary>
     public static DateTimeOffset AddNanoseconds(this DateTimeOffset self, long nanoseconds)
     {
-        if (nanoseconds == 0)
-        {
-            return self;
-        }
-
         return self + nanoseconds.Nanoseconds();
     }
 
@@ -299,11 +289,6 @@ public static int Microsecond(this DateTimeOffset self)
     /// </summary>
     public static DateTime AddMicroseconds(this DateTime self, long microseconds)
     {
-        if (microseconds == 0)
-        {
-            return self;
-        }
-
         return self + microseconds.Microseconds();
     }
 
@@ -312,11 +297,6 @@ public static DateTime AddMicroseconds(this DateTime self, long microseconds)
     /// </summary>
     public static DateTimeOffset AddMicroseconds(this DateTimeOffset self, long microseconds)
     {
-        if (microseconds == 0)
-        {
-            return self;
-        }
-
         return self + microseconds.Microseconds();
     }