Changes needed for IFNA & IFERROR function definitions for empty cells

For both IFNA & IFERROR, The function definitions currently state:

Semantics: Computes X. If ISNA(X) is true, return Alternative, else return X. Thus this is semantically equivalent to IF(ISNA(X); Alternative; X), except that X is only computed once. If X or Alternative is a cell reference to an empty cell, it is treated as the empty string "".

In both Open Office & Excel, the semantic equialent listed above, returns a zero, not an empty string. The function definition needs to be updated to allow for this.