Fixed size of symbols in legends if size is calculated

[lgoltz]

If the size of a symbol is calculated, e.g.:

<Size>
  <ogc:Function name="idiv">
    <ogc:Literal>50000</ogc:Literal>
    <ogc:Function name="GetCurrentScale" />
  </ogc:Function>
</Size>

The legend created by deegree creates not readable legends:

This PR fixes this by using a default size of 6 if the size of the symbol is -1.

Created legend after the fix:

[julianzz98]

Change/fix was successfully tested on a local environment!

See results (without transparency):