Portfolio Shortcode: Warnings on WordPress.com causing issues with some themes

When using a portfolio shortcode on a WordPress.com sandbox, warnings are displayed before the projects (full screenshot of the warning available on the theme ticket linked below).

The warning doesn't display on self-hosted sites, and is not visible on non-sandboxed sites.

#### Steps to reproduce the issue

1. On a sandboxed WordPress.com site running a standard theme (I used Radcliffe 2), add some portfolio projects.
2. Add the `[portfolio]` shortcode to the page, with or without specific attributes.
3. Save and publish the page.
4. View the page.

#### What I expected

Portfolios to display, but without warnings.

#### What happened instead

A warning displayed before the projects.

Came up while testing 2141-wpcom-premium-themes (includes screenshot).

Start by reproducing the [portfolio] shortcode on a sandboxed WordPress.com site using the Radcliffe 2 theme, comparing it with self-hosted and non-sandboxed sites. Trace the shortcode entry point and use the linked theme-ticket screenshot to identify the warning; done means portfolio projects render without warnings in the affected environment.

Written by the indexing model from the issue text.