fix: DIA-1123: Config with empty string value breaks parser

[hakan458]

When instantiating a LabelInterface object with a config that included a Label tag with empty string value, the parser would break E.g.

<View>
  <Text name="text" value="$text"/>
  <Labels name="type" toName="text">
    <Label value="" />
    <Label value="" />
  </Labels>
</View>

We allow empty string as a value in validation it seems, but parser didnt handle it