Use nil instead of false to disable callable settings

[st0012]

Using false to disable settings doesn't match nowadays convention. So I'm changing the disabling value of before_send and async (both inherited from sentry-raven) from false to nil.
Since they are both disabled by default, most users shouldn't need to disable them manually anyway. So I think this change should have a minimum impact and can be shipped in a minor release.

[codecov-commenter]

Codecov Report

Merging #1594 (c04d2af) into master (17836c4) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1594      +/-   ##
==========================================
+ Coverage   98.43%   98.46%   +0.02%     
==========================================
  Files         129      129              
  Lines        7210     7218       +8     
==========================================
+ Hits         7097     7107      +10     
+ Misses        113      111       -2     

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/configuration.rb	98.99% <100.00%> (+1.00%)	⬆️
...ntry-ruby/spec/sentry/client/event_sending_spec.rb	99.53% <100.00%> (ø)
sentry-ruby/spec/sentry/configuration_spec.rb	100.00% <100.00%> (ø)

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