Refactor ConversionAdd to expression-bodied member

Simplified the `ConversionAdd` method by converting it from a block body to an expression body. Removed the explicit null check for the `other` parameter, potentially relying on other mechanisms to ensure `other` is not null. The method now directly returns the result of `Create(Quantity + other)`.

Author: matt-edmondson

PhysicalQuantity/Quantities/PhysicalQuantity.cs

@@ -63,12 +63,7 @@ public TSelf Clamp<T1, T2>(T1 min, T2 max)
 		where T2 : INumber<T2>
 		=> Create(Quantity.Clamp(min.ToSignificantNumber(), max.ToSignificantNumber()));
 
-	protected TSelf ConversionAdd(SignificantNumber other)
-	{
-		ArgumentNullException.ThrowIfNull(other);
-
-		return Create(Quantity + other);
-	}
+	protected TSelf ConversionAdd(SignificantNumber other) => Create(Quantity + other);
 }
 
 /// <summary>