fix for self-closing tags

[davidgith1]

Initally proposed this fix here but this fix makes more sense in the lib.

My previous explanation:

I'm using koreader + comicmeta on Android.

1. Have a cbz file which has a ComicInfo.xml file that looks like f.e. this:

<?xml version="1.0" encoding="utf-8"?>
<ComicInfo><Title>Ruri Dragon - Ch.48</Title><Series>Ruri Dragon</Series><Number>48</Number><Count>0</Count><Volume>0</Volume><AlternateSeries/><AlternateNumber/><AlternateCount>0</AlternateCount><Summary/><Notes/><Year>0</Year><Month>0</Month><Day>0</Day><Writer/><Penciller/><Inker/><Colorist/><Letterer/><CoverArtist/><Editor/><Translator/><Publisher/><Imprint/><Genre/><Tags/><Web/><PageCount>19</PageCount><LanguageIso/><Format/><BlackAndWhite/><Manga>YesAndRightToLeft</Manga><Characters/><Teams/><Locations/><ScanInformation/><StoryArc/><StoryArcNumber/><SeriesGroup/><AgeRating/><MainCharacterOrTeam/><Review/><Gtin/></ComicInfo>

The important part is that is has at least one self closing tag:

<ComicInfo>
<Title>Chapter 1</Title>
<Writer/>
</ComicInfo>

2. Navigate to the folder of that cbz file and trigger metadata extraction (menu → "Extract Comic Meta").
3. The extraction will start but when it finishes it will say: Comic metadata extraction complete. Successfully extracted 0 / 1
   No errors and nothing extracted.

After this fix, a ComicInfo.xml like the examples above will result in a successful extraction.

The self-closing tags come back as tables so it fails when the plugin is expecting a string. This fix make them come back as an empty string instead.