HPPC-5417 drop not null constraint for duration

What does this MR do?

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Merge request java checklist

• Self review is complete by author
• Merge request title is correct and is clear. i.e. HPPC-0000 added bank details screen
• Relevant milestone has been set
• On merge commits are squashed (check if enabled)
• Build is passing
• If the MR starts with NO-JIRA there are no functional changes that would cause behaviour to be any different
• Coding standards are https://bsa2468.atlassian.net/wiki/spaces/NCH/pages/2218197003/Code+Quality+Standard+1.1+DRAFT+-+CL and follows google styling https://google.github.io/styleguide/javaguide.html
• There are no additional changes other than the story to be covered
• The commit message is named correctly and is clear to understand i.e. HPPC-0000 added api endpoint should be HPPC-0000 added api endpoint to update bank details
• The code matches the Acs defined in the story
• There are no additional changes covered other than the Acs mentioned

Others

• The README has been updated and is in clear english without any gramatical errors is correct, no use of tabs

Closes HPPC-5417