Fix divide by zero issues on Trend indicator

[philippjfr]

Fixes #2146

[codecov]

Codecov Report

Merging #2148 (f07f712) into master (31b5350) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #2148      +/-   ##
==========================================
- Coverage   83.81%   83.78%   -0.04%     
==========================================
  Files         182      182              
  Lines       21672    21672              
==========================================
- Hits        18165    18157       -8     
- Misses       3507     3515       +8     

Impacted Files	Coverage Δ
panel/widgets/indicators.py	94.83% <100.00%> (ø)
panel/tests/pane/test_base.py	88.23% <0.00%> (-11.77%)	⬇️
panel/io/reload.py	64.36% <0.00%> (-2.30%)	⬇️
panel/pane/base.py	87.89% <0.00%> (-0.90%)	⬇️

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