feat: allow/fix no keywords in output.xml processing

[HuntTheSun]

Hi @timdegroot1996

for the sake of our pipeline and storage systems, we strip our output.xml of keywords during robotframework tests.
Sadly, robotframework-dashboard runs into an exception here if there are no keywords in the output:

# processors.py function calculate_keyword_averages
run_start = keyword_list[0][0] # keyword list is None if no keywords in xml

Would my "fix" be alright or do you dislike the "feature"/"fix"?
The keywords are empty for the run in the web-ui, no errors in the console.

Sorry for opening a PR directly, I thought a new Issue for a 1-line change would be a bit overkill :)

[timdegroot1996]

@HuntTheSun it is indeed fine not to make an issue for this. I think it is indeed small enough.

I dont mind adding this, but I think there could be 2 improvements:

1. If we do this for the keywords, would it not make sense to also do this for the other data types as well? So also for test, suite and run?
2. If we add it for all types, or even only for keywords, I think it's a good idea to still show a warning in the console during the processing that this has occurred. This way it is not some invisible handling but it can be found if people look for it.

Let me know what you think about these points!