Fixes instability issue in update_with_missing that could result in NaN

[tunnellm]

In some edge cases, one or more instances of the diagonal value of the WtW matrix could be zero leading to the propagation of NaNs. This is not a problem in the update function where a stability constant, TINY_NUM is added to the diagonal values--but this was left out in the update_with_missing function. This fix adds this to the update_with_missing function.