function getSlotProjections(htmlElement) {
    return isSlot(htmlElement) ? /** @type {Element[]}  */ (htmlElement.assignedElements()) : [];
  }
Returns a slot projection or it returns `null` if `htmlElement` is not an `HTMLSlotElement`
@example
Let's say this is a custom element declared as follows:
```
<custom-element>
  shadowRoot
    <div id="dialog-wrapper">
      <div id="dialog-header">Header</div>
      <div id="dialog-content">
        <slot id="dialog-content-slot" name="content"></slot>
      </div>
    </div>
  <!-- Light DOM -->
  <div id="my-slot-content" slot="content">my content</div>
</custom-element>
```
Then for `slot#dialog-content-slot` which is defined in the ShadowDom the function returns `div#my-slot-content` which is defined in the LightDom
@param {HTMLElement|HTMLSlotElement} htmlElement
@returns {Element[]}