handles field_get_items() FALSE values

node_convert_field_convert uses field_get_items to retrieves values.
Anyway field_get_items can return FALSE (when the field is an empty array).
A typical case is an optional term reference where "no value" means not even a [LANGUAGE_NONE] indexed array, the field is only an empty array.
In such a case, we don't want to artificially create index because chances are that they will be populated in unpredictable (and buggy way) (at least in ways that I don't understand).

In the taxonomy example, you will end up with ["und"][0]["tid"] => NULL but it's wrong, and the taxonomy module will then complain about Integrity constraint violation: 1048 Column tid cannot be null

The attached patch may solve this problem but it needs understanding (and probably testing too).