Bug Report

Description

When using image presets with defined sizes in nuxt.config.ts, the preset sizes are not applied if the component's sizes prop is undefined. This causes the image to fallback to density-based srcset (1x, 2x) instead of the expected width-based srcset (320w, 640w, etc.), leading to incorrect image serving.

Reproduction

// nuxt.config.ts
export default defineNuxtConfig({
  image: {
    presets: {
      responsive: {
        sizes: '100vw sm:50vw md:400px'
      }
    }
  }
})

<!-- Component usage -->
<NuxtImg 
  src="/image.png" 
  preset="responsive"
  :sizes="undefined"  // or simply omitted
  width="300" 
  height="400" 
/>