Couple of bugfixes after testing function with a different signature.
1. When <globals> input column doesn't exist, script fails
2. If user missed some cells, they're not read as blank values and hence
the sizes of value arrays differ from the actual number of test cases.

For bug#1, the function that returned a string representation of globals
macro definitions is fixed. Now it will return "" so the corresponding
tag is wiped out from the resultant autogenerated file

For bug#2, some more assert checks are added in ScriptGenerator
constructor, so that spreadsheet editing errors can be rectified
beforehand