get_the_excerpt() deprecated arg warning

[tomusborne]

We get a deprecated arg warning when $source_id doesn't return an actual ID in our get_the_excerpt() function: https://github.com/tomusborne/generateblocks/blob/8816e5ab471cdb5b6617e1d8ab9927bc0165a689/includes/class-dynamic-content.php#L205

Function get_the_excerpt was called with an argument that is <strong>deprecated</strong> since version 2.3.0 with no alternative available.

Reference: https://secure.helpscout.net/conversation/2060989166/360161?folderId=3610635