Field undefined extended sub-property protection

[saverett]

Use ng2 elvis-operator style protection for data field sub-properties that could be undefined to help prevent Cannot read property 'X' of undefined exceptions.

For example:
In the following datatable column declaration, primeng datatables would throw a Cannot read property 'c' of undefined exception if b was undefined. It is often the case that extended field sub-properties are present for some collections/rows, and not present for others. This PR solves this problem, while still allowing primeng datatables' built-in features that rely on the fully-qualified field property (such as sorting and filtering) to work correctly:

<p-column field="a.b.c" header="C"></p-column>

[cagataycivici]

Thank you.