Always prefer return this in constructor early returns

This is a “just in case” commit where early ES2015 class implementations would return `undefined` in a subclass constructor (extended component) rather than the subclass instance

Might only affect Safari 9

See: #2987 (comment)

Author: colinrotherham

src/govuk/components/character-count/character-count.mjs

@@ -116,7 +116,7 @@ export class CharacterCount {
     } else if ('maxlength' in this.config && this.config.maxlength) {
       this.maxLength = this.config.maxlength
     } else {
-      return
+      return this
     }
 
     /** @private */

src/govuk/components/error-summary/error-summary.mjs

@@ -21,8 +21,6 @@ export class ErrorSummary {
     // we need to safeguard against this so things keep
     // working the same now we read the elements data attributes
     if (!($module instanceof HTMLElement)) {
-      // Little safety in case code gets ported as-is
-      // into and ES2015 class constructor, where the return value matters
       return this
     }