You have resigned and obtained no due certificate 

if you disclose it would not create problems in your new job 

As you have been released from previous job before joining new one there will be no legal issue. You can disclose the fact of your previous employment to new employer. 

You there is requirements regarding your work experience in the new job than you can tell them about your previous experience otherwise not required.

No, not at all , if you wants to use the experience then fill it up in the form to be issued at the time of interview, if any , if not , then when you join then fill it up or you may disclosed if asked for to the interviewer if not then why to worry.

No it will not create as after filing the form you joined and resigned and you can disclose same at time of joining . As you have release and No dues letter. 

Whatever status is filled in form should of that time when the form has been filed and to last date of correction of error in form. You can inform them, won`t effect your job. 

I don't understand why you want to disclose it now.  In my opinion, there is no need to disclose it when you have already resigned and relieved before joining.  Start as a new beginning and do not share it with anybody after joining.

Do not begin your service life with false information. i have seen people suffered termination for providing false information in their mid-carieer, some even after a long span of service. You will fall into your own net as a prey.

Since you were unemployed at the time of appearing for vrious examinations for this job, you need not worry about the subsequent employment  elsewhere. You must be concerned only about resigning the current job properly as per procedures and get a proper relieving letter before joining the new employment.

If you can conceal it's good.  If they ask you, you will not have any other choice other than to reveal.  They will not harrass you for noc. 

Dear Client

No this is not a big issue because at time of filling of form you have provided the correct Information about you and you can fill up the future events in a form you are filling today.

so if you disclose the previous employment at time of joining the new job there will not be any issue with new employer. but don't try to hide the information otherwise it can create problem to you.