don't set undefined input value

[Rich-Harris]

This fixes #1233. Instead of blindly setting input.value to its binding value, the value is only set if it's not undefined or null (or if the value was previously truthy).

This way, Firefox doesn't present a newly-created <input> as invalid when it hasn't had a value set yet (for this to work, the value must initially be undefined rather than "").

[codecov-io]

Codecov Report

Merging #3430 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #3430   +/-   ##
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  Coverage   50.25%   50.25%           
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  Files           1        1           
  Lines         197      197           
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  Hits           99       99           
  Misses         98       98

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[DmitryMyadzelets]

This merge is somehow broken in the current master (Svelte v3.44.2):

export function set_input_value(input, value) {
	input.value = value == null ? '' : value;
}

The fix was:

export function set_input_value(input, value) {
	if (value != null || input.value) {
		input.value = value;
	}
}

So the issues #4725 and #1233 take place again.

Update: The fix wasn't correct, btw. If the value is 0 or "" it wouldn't set input.value.

Update: Commits where the issue evolves: 081f7cd, d8fb0bb

I'm not aware why null is a special case for the input value. I experience this issue when the values are not defined but required when the <form> is loaded. It seem a common default state. I'd propose the next fix:

export function set_input_value(input, value) {
	if (value != null && value != undefined) {
		input.value = value;
	}
}

However, this breaks intentional assigning of undefined to the values. Then, we have to avoid setting input.value in case it's not defined in the initial state only (mounting?).