PR for v.1.9.2

[hcpchris]

• BUGFIX: Resolved over-broad False-y validation of PieDataLabel.distance to allow for 0
  values. Closes PieDataLabel.distance does not support a distance of 0 #192.
• BUGFIX: Fixed over-broad False-y validation of Tooltip.header_format to allow for empty string
  values, bringing it in line with Tooltip.footer_format. Closes Tooltip.header_format validation not consistent with Tooltip.footer_format #193.

[codecov]

Codecov Report

Attention: Patch coverage is 16.66667% with 5 lines in your changes missing coverage. Please review.

Project coverage is 87.98%. Comparing base (0f3a4af) to head (9d37de4).

Files	Patch %	Lines
highcharts_core/utility_classes/data_labels.py	0.00%	2 Missing and 1 partial ⚠️
highcharts_core/options/tooltips.py	33.33%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #194      +/-   ##
==========================================
- Coverage   88.00%   87.98%   -0.02%     
==========================================
  Files         210      210              
  Lines       31151    31155       +4     
  Branches     2547     2549       +2     
==========================================
  Hits        27413    27413              
- Misses       2780     2783       +3     
- Partials      958      959       +1     

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