ZeroDivisionError in kraken module

[davidealbanese]

Description of bug:
MultiQC returns a Python ZeroDivisionError on some kraken2 reports.

MultiQC Error log:

Module kraken raised an exception: Traceback (most recent call last):
  File "/usr/lib/python3.8/site-packages/multiqc/multiqc.py", line 569, in run
    output = mod()
  File "/usr/lib/python3.8/site-packages/multiqc/modules/kraken/kraken.py", line 60, in __init__
    self.sum_sample_counts()
  File "/usr/lib/python3.8/site-packages/multiqc/modules/kraken/kraken.py", line 134, in sum_sample_counts
    self.kraken_sample_total_readcounts[s_name] = round(float(row['counts_rooted']) / (row['percent'] / 100.0))
ZeroDivisionError: float division by zero

File that triggers the error:
kraken_reports.tar.gz

MultiQC run details (please complete the following):

• Command used to run MultiQC: multiqc .
• MultiQC Version: 1.9
• Operating System: NA (singularity shell docker://ewels/multiqc:1.9)
• Python Version: NA
• Method of MultiQC installation: NA

Additional context

[davidealbanese]

Related to #1276.