wr2_kostalsteca errors out with unhandled exception when <Yields/> in yields.xml is empty

[andlem74]

In case of my inverter, the yields.xml looks like this:
<root> <Device Name="PIKO 3.6 MP" Type="Inverter" Serial="755810BL006202240015" BusAddress="1" NetBiosName="INV006202240015" IpAddress="192.168.0.119" DateTime="2022-06-29T14:27:27"> <Yields/> </Device> </root>
The code in wr2_kostalsteca tries to read data from the empty Yields which fails. Please change the code to handle this condition by returning NAN or 0 for the yield instead of raising an unhandled exception.

[andlem74]

Fix merged into master branch.