Area tag does not give alt="" as of dot 7.0.2

Graphviz dot does not write out `alt=""` with the `area` tag as of version 7.0.2, the attribute should be IMPLIED / optional.

From https://gitlab.com/graphviz/graphviz/-/issues/265 and https://gitlab.com/graphviz/graphviz/-/issues/2319

On the page https://www.w3schools.com/tags/tag_area.asp I see under Attributes:
> | alt  | text  | Specifies an alternate text for the area. Required if the href attribute is present |

Also in the file https://www.w3.org/TR/2018/SPSD-xhtml1-20180327/DTD/xhtml1-strict.dtd I see:
```
<!ELEMENT area EMPTY>
<!ATTLIST area
  %attrs;
  %focus;
  shape       %Shape;        "rect"
  coords      %Coords;       #IMPLIED
  href        %URI;          #IMPLIED
  nohref      (nohref)       #IMPLIED
  alt         %Text;         #REQUIRED
  >
```
and in https://www.w3.org/TR/2018/SPSD-xhtml1-20180327/DTD/xhtml1-transitional.dtd I see:
```
<!ELEMENT area EMPTY>
<!ATTLIST area
  %attrs;
  %focus;
  shape       %Shape;        "rect"
  coords      %Coords;       #IMPLIED
  href        %URI;          #IMPLIED
  nohref      (nohref)       #IMPLIED
  alt         %Text;         #REQUIRED
  target      %FrameTarget;  #IMPLIED
  >
```

Author: albert-github

src/dotfilepatcher.cpp

@@ -233,6 +233,14 @@ bool DotFilePatcher::convertMapFile(TextStream &t,const QCString &mapName,
     if (buf.startsWith("<area"))
     {
       QCString replBuf = replaceRef(buf,relPath,urlOnly,context);
+      // in dot version 7.0.2 the alt attribute is, incorrectly, removed.
+      // see https://gitlab.com/graphviz/graphviz/-/issues/265
+      int indexA = replBuf.find("alt=");
+      if (indexA == -1)
+      {
+        replBuf = replBuf.left(5) + " alt=\"\"" + replBuf.right(replBuf.length() - 5);
+      }
+
       // strip id="..." from replBuf since the id's are not needed and not unique.
       int indexS = replBuf.find("id=\""), indexE;
       if (indexS>0 && (indexE=replBuf.find('"',indexS+4))!=-1)