fix(gatsby-transformer-sharp): add base64 field as non-null in schema

[Kerumen]

Description

Follow up of #20314, I added the base64 field because the fragment defines it and it avoids TS errors.

[vladar]

Thanks so much for the PR!

But base64 can actually be null:

gatsby/packages/gatsby-plugin-sharp/src/index.js

Lines 608 to 610 in ea65aea

	return {
	base64: base64Image && base64Image.src,
	aspectRatio: images[0].aspectRatio,

We have a separate option base64 for fluid and fixed methods which controls this (when set to false, base64 output will be null.

And gatsby-image is handling null-case explicitly:

gatsby/packages/gatsby-image/src/index.js

Lines 504 to 513 in 8a82ce8

	{/* Show the blurry base64 image. */}
	{image.base64 && (
	<Placeholder
	ariaHidden
	src={image.base64}
	spreadProps={placeholderImageProps}
	imageVariants={imageVariants}
	generateSources={generateBase64Sources}
	/>
	)}

So I incline to close this PR unless I am missing something?

[Kerumen]

Oh yes. And I double checked gatsby-image definition file and base64 is defined as optional.

Sorry for the trouble!