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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