fix(fast-html): a hydrating repeat with no items has no hydration markers #7211
Description
🐛 Bug Report
When a repeat (f-repeat) is present in the template but the stale/initial state has no items, there's no way to know where the repeat should bind to, because there are no hydration markers to indicate its mapped position.
💻 Repro or Code Sample
<test-element>
<template shadowrootmode="open">
<!--fe-b$$start$$0$$f-when-binding$$fe-b--><!--fe-b$$end$$0$$f-when-binding$$fe-b-->
</template>
</test-element>
<f-template name="test-element">
<template>
<f-when value="{items}">
<f-repeat value="{item in items}">{{item}}</f-repeat>
</f-when>
</template>
</f-template>
<test-element-breaks>
<template shadowrootmode="open">
</template>
</test-element-breaks>
<f-template name="test-element-breaks">
<template>
<f-repeat value="{item in items}">{{item}}</f-repeat>
</template>
</f-template>🤔 Expected Behavior
Both components would render without hydration errors.
😯 Current Behavior
The test-element renders because the f-repeat is guarded by the wrapping f-when.
The test-element-breaks component fails with a hydration error.