update title match regex

[PrajwolAmatya]

Description

Current regex to get the title works when there is heading present.

## Slide Title ::slide:title-content
slide content

We get the title of the slide as Slide Title.

But as introduction of the new slide template for full image background, we provide the content as:

## ::slide:image
slide content

Here, the title of the slide is returned as slide content, which is not true.

This PR fixes this issue, and returns empty string when there is no title present for this slide.

[nirajacharya2]

if we return empty string the toc will have a item without any text. we need to fix that too

[PrajwolAmatya]

if we return empty string the toc will have a item without any text. we need to fix that too

Thanks for noticing this issue. 👍
This issue is from the templates side. I have fixed it in PR opf/revealjs-awesoMD-templates#5