Add space to empty anchor tags to fix internal links.

[rpedela]

Internal links (anchor tags) are not linked correctly when the anchor's content is empty.
This adds a space with a height of 0 when it is empty so that internal links are linked correctly.
Fixes issue #1554.

This PR targets master rather than 0.12.3.x like #2949 did.

I tested the following scenarios:

<a id="abc"></a>
<a id="abc" href=""></a>
<a name="abc"></a>
<a name="abc" href=""></a>
<div id="abc"></div>
<div name="abc"></div>

[rpedela]

The following code was added based on the assumption that <a id="abc"></a> should be handled identically to <a name="abc"></a>. Is that a safe assumption?

QString id=elm.attribute("id");
if (id.isEmpty()) id=elm.attribute("ns0:id");
if (id.startsWith("__WKANCHOR_")) anchors[id] = elm;

[rpedela]

@ashkulz ready for review. I think I did all the things you asked for like targeting master, etc.

[ashkulz]

@rpedela: won't have time for a week or so, sorry about that.

[akerouanton]

@ashkulz Could you take a look at this PR? It's been a while and some people still face this issue.