or
>>> s = ["1" ,"2", "", "abc"]
>>> [int(i) if i and i.isdigit() else -9999 for i in s]
[1, 2, -9999, -9999]
>>>
can you test
[int(i) if i and i.isdigit() else -9999 for i in !FieldName!][0] or
( int(i) if i and i.isdigit() else -9999 for i in !FieldName! )
no arc* to text