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

Hi

- 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