Enable use_if_null_to_convert_nulls_to_bools lint rule (dart-lang/markdown#417)

pkgs/markdown/analysis_options.yaml

diff --git a/pkgs/markdown/analysis_options.yaml b/pkgs/markdown/analysis_options.yaml
index 76d6155..18598d8 100644
--- a/pkgs/markdown/analysis_options.yaml
+++ b/pkgs/markdown/analysis_options.yaml
@@ -32,4 +32,5 @@
     - test_types_in_equals
     - throw_in_finally
     - prefer_final_locals
+    - use_if_null_to_convert_nulls_to_bools
     - prefer_interpolation_to_compose_strings