Update InlineInput to allow undefined `value` prop

[yaelleC]

Description

Currently, the InlineInput has a compulsory prop value that is a string. If we want to render an input that has no value, we have to send it an empty string. It would be best to have the option to pass undefined or no value at all

Code

https://github.com/grafana/grafana-aws-sdk-react/blob/1d744d0937499a0a89809df89d820c0046af2cfa/src/sql/ConfigEditor/InlineInput.tsx#L7-L16

Acceptance criteria:

• [ ] The following code can be ran even if clusterIdentifier does not exist:

  <InlineInput
      {...props}
      value={props.options.jsonData.clusterIdentifier}
  />

[fridgepoet]

We've decided not to work on this one for now.