fix(homepage): placeholder inputs

[seaerchin]

Problem

Previously, we always prefilled sections on initial add - this allows people to slip up and accidentally click save even when it was the default values.

Closes IS-500

Solution

• Separate prefill values and placeholder values. This then allows us to add an empty block (on left panel) while still displaying a sane preview value on the preview panel. This was done through first filtering by which properties have an empty value then merging with default values on render.
• fix existing validation - we always check if length < 0, which will never be true.
• extract stuff out from current validation to reuse - this was because the validation also sets some additional properties on an error object.
• write util function to compute hasError from the formState - this is required because errors are only set on change. this means that, for example, users can add an empty block and save instantly (as no change was made), which runs counter to what we want.
• add onBlur = onChange. as the value is controlled by us and there's no value change on blur, this only serves to retrigger validation on blur events so that we inform the user about what failed validation (empty input on initial create for eg)

Tests

• trigger validation on existing items
  • this can be done through exceeding max limit or deleting stuff on required fields
• create a new block
• save should be disabled
• expand accordion
• click on input
• click away
• error message should pop up
• save should still be disabled

Notes

Feels like we're reinventing the wheel here already. i don't want to put more effort into this but our validation strategy feels abit off as closing the accordion, for example, does not trigger validation.

maybe can consider shifting to a form lib that does this for us but is a significant effort so i think we should just live with this as an intermediate measure until we decide that we want to nudge website builders in the right dir

@sehyunidaaa - maybe we can prompt the isOptional already otherwise feels like users might grapple with this

[alexanderleegs]

Is there a reason why we pass in hero section this way instead of like the other preview sections? (destructuring existing object so that only non-blank params will overwrite)

[seaerchin]

don't the other preview adopt this pattern also?

[seaerchin]

oh cos this way was less work - i'm open to keeping it consistent tbh

[seaerchin]

this is abit annoying to do here cos we have to do a type cast + overwrite a child array. lmk if this is a blocker for u

[alexanderleegs]

I'm okay with leaving it as is if it's annoying to implement!

[QiluXie]

lgtm, thanks @seaerchin for addressing the comments quickly