'sandbox' attribute is not removed when bound to a nullish value #13946
Description
Version
3.5.22
Reproduction link
Steps to reproduce
Run the reproduction and confirm the presence of sandbox attribute in the iframe element.
What is expected?
According to the Vue docs, the sandbox attribute shouldn't be present - the sandbox restrictions shouldn't be applied.
This behavior is observable in Vue 2 in this example:
https://codepen.io/tomasz-pucka/pen/gbPrzoV
What is actually happening?
sandbox attribute is set to 'null' string resulting in console errors. Setting it to an empty string applies the most restrictive sandbox mode.