fix updating addlink_db_table with null values.

[alireza-delavari]

as in this comment mentioned: #1020 (comment)

after enabling "Proxy" checkbox in properties for any link item in table and clicking on OK button,
disabling "Proxy" checkbox wont save anymore.

its also same for other fields, for example setting "Referrer" field to empty wont save.

it was because of "updateAddLinkTable" method in "data_base.py" ,
when updating db table it was setting new null values to its previous stored value in db table.
so we couldn't update anything to null anymore.

[alireza-amirsamimi]

Hello
As I said there, I don't have this problem :)
I can't confirm this bug.

[alireza-delavari]

did you fill IP and Port field when enabling proxy checkbox?

I filmed my steps to make the bug more clear:
setting proxy check box enable, fill ip and port field, click ok,
then setting proxy check box disable click ok
, then open properties dialog again, proxy still is enable
persepolis_1_proxy.webm

its the same for other fields:
persepolis_2_referer.webm

[alireza-amirsamimi]

Thank you 👍