[Bug]: Default image placeholder's src attribute generated / exported as <svg> element

Hi, and thanks for the great library !

We might have found the following bug when playing around with it:

When adding a new default "Image Block" without further configuration (leaving the default image placeholder), the exported / generated HTML code embeds the default svg element as the image's src attribute value (cf attached screenshots).

After digging a bit into the code, it appears that the grapesjs/src/dom_components/model/ComponentImage.js's getAttrToHTML function should call this.getSrcResult (not sure whether or not to use the fallback option) rather than this.get('src') to retrieve the base64 encoded image src attribute (cf https://github.com/artf/grapesjs/blob/dev/src/dom_components/model/ComponentImage.js#L78) .

Note: this would not satisfy a case where the embedAsBase64 option would be set to false, maybe in this case a public image placeholder URL could be used (TBD) ?

I'd be happy to open a PR to fix it, let me know :)