Fix of images handling of HTML embedded images which cause NullPointerException (#1436)

[speckyspooky]

Fix of images which are embedded like URL on an html document which is embedded into a birt report element (e.g. dynamic text)

[speckyspooky]

@wimjongman
Hello Wim, this is a fix which should be included into BIRT4.14 to avoid the according exception.

By the way, I plan no further checkin for 4.14 only if we should get some bug notifications.
Other changes will be moved after the release of 4.14 (e.g. change of the font-mapping).

[reiner-killet]

Is fix #1440 contained in the nightly build (https://download.eclipse.org/birt/updates/nightly/latest/index.html)?
I downloaded the RCP designer for Windows from there and the same error message still occurs.

[speckyspooky]

The fix is currently not included to the nightly built because I wait of the approval of @wimjongman
If he has done the quality check I will consider his response and with the approval I can merge it to the master and afterwards it will go into the nightly built.

[wimjongman]

Looks good to me, Thomas.