Improved checking of null vector elements.

The suggestions by Sébastien have been added and the second
implementation of FieldVector (SparseFieldVector) has been adapted
accordingly, despite it is deprecated.

JIRA: MATH-861

git-svn-id: https://svn.apache.org/repos/asf/commons/proper/math/trunk@1455233 13f79535-47bb-0310-9956-ffa450edef68