Stop byteslicing empty strings in breadcrumbs

[nirebu]

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Description

Describe your changes:

If we don't pass a message to the breadcrumb message, we default to an empty string. However we can avoid byteslicing it, which allocates a new empty string every time regardless.

This is a pretty hot path, and the tests pass. Only difference I can think of is returning a frozen empty string, which could cause issues downstream. Let me know if it should be dup-ed!

require 'benchmark-memory'

MAX_MESSAGE_SIZE_IN_BYTES = 1024 * 8

def current(message)
  (message || "").byteslice(0..MAX_MESSAGE_SIZE_IN_BYTES)
end

def updated(message)
  message ? message.byteslice(0..MAX_MESSAGE_SIZE_IN_BYTES) : ""
end

Benchmark.memory do |x|
  x.report("current") do
    current(nil)
  end

  x.report("updated") do
    updated(nil)
  end

  x.compare!
end

Calculating -------------------------------------
current 80.000 memsize ( 0.000 retained)
2.000 objects ( 0.000 retained)
1.000 strings ( 0.000 retained)
updated 0.000 memsize ( 0.000 retained)
0.000 objects ( 0.000 retained)
0.000 strings ( 0.000 retained)

Comparison:
updated: 0 allocated
current: 80 allocated - Infx more

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 64.52%. Comparing base (db975df) to head (23eeaba).
Report is 9 commits behind head on master.

Files with missing lines	Patch %	Lines
sentry-ruby/lib/sentry/breadcrumb.rb	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2574      +/-   ##
==========================================
+ Coverage   64.11%   64.52%   +0.40%     
==========================================
  Files         123      123              
  Lines        4724     4679      -45     
==========================================
- Hits         3029     3019      -10     
+ Misses       1695     1660      -35     

Components	Coverage Δ
sentry-ruby	60.17% <81.81%> (+0.32%)	⬆️
sentry-rails	59.82% <ø> (+0.60%)	⬆️
sentry-sidekiq	97.16% <ø> (ø)
sentry-resque	94.44% <100.00%> (+1.58%)	⬆️
sentry-delayed_job	95.65% <ø> (ø)
sentry-opentelemetry	99.31% <ø> (ø)

Files with missing lines	Coverage Δ
sentry-ruby/lib/sentry/breadcrumb.rb	46.87% <0.00%> (+1.42%)	⬆️

... and 34 files with indirect coverage changes

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