[pgAdmin4][Patch]: Fix RM1790 - [Web] Support setting a field's value to "null"

Hi

*Issue:*
- On viewing table data, If we edit a column and set value of column(type:
text) to "null", It always takes it as empty string. It doesn't honour null
values.

Solution:
- Take a flag "*is_null*" for columns with data type 'text', then on GUI,
whilst user edits a text field, an additional option with checkbox(is_null
?) is given to take null values. If checkbox is checked, on JS side we
check "*is_null*" flag and pass field value to null if selected.

Please find patch and review.

Thanks,
Surinder Kumar