The magnitude of anything is never negative. It can be even seen from the formula that the components are squared. A squared value can never be negative. Even if the component is negative the square will be always positive.
So, magnitude of the vector is <u>not</u> negative.
As we can see that the quantity on the right side of equation is always positive since square root of a quantity is always positive thus we conclude that the magnitude of any vector and hence vector A is always positive.