jsonformat: Only set the 'linecount' if already present

[lowell80]

Don't update/set the linecount when updating _raw unless the field is already contains a numeric value. If not, don't create it. (Avoid creating the

For example, if you have the search:

index=* b635172e-2cd9-45b7-adc8-4efb14d21901 | table _time _raw sourcetype source | jsonformat

We should haven't to explicitly then re-remove the lincount field. In other words, it makes sense to keep it when it's present, but otherwise don't get in the way.