Only create an array with default options if we have default options

If the options passed in don't have a default key, there's no point in
creating an array from those empty results when we can just go straight
to creating an empty array.

Benchmarks:
```ruby
master_version with false
{:FREE=>-2497, :T_STRING=>52, :T_ARRAY=>2000, :T_HASH=>1000, :T_IMEMO=>1}
master_version with true
{:FREE=>-3001, :T_ARRAY=>2000, :T_HASH=>1000}
fast_version with false
{:FREE=>-1001, :T_ARRAY=>1000}
fast_version with true
{:FREE=>-3001, :T_ARRAY=>2000, :T_HASH=>1000}
Warming up --------------------------------------
master_version with false
                       104.985k i/100ms
master_version with true
                       118.737k i/100ms
fast_version with false
                       206.013k i/100ms
fast_version with true
                       107.005k i/100ms
Calculating -------------------------------------
master_version with false
                          1.970M (±24.6%) i/s -      8.924M in   5.010302s
master_version with true
                          2.152M (±12.4%) i/s -     10.686M in   5.051588s
fast_version with false
                          5.613M (±19.6%) i/s -     26.782M in   5.003740s
fast_version with true
                          2.027M (±15.8%) i/s -      9.951M in   5.065670s

Comparison:
fast_version with false:  5613159.2 i/s
master_version with true:  2152354.4 i/s - 2.61x  slower
fast_version with true:  2027296.0 i/s - 2.77x  slower
master_version with false:  1969824.9 i/s - 2.85x  slower
```

Benchmark code:
```ruby
begin
  require "bundler/inline"
rescue LoadError => e
  $stderr.puts "Bundler version 1.10 or later is required. Please update
                your Bundler"
  raise e
end

gemfile(true) do
  source "https://rubygems.org"

  gem "benchmark-ips"
  gem "rails"
end

def allocate_count
  GC.disable
  before = ObjectSpace.count_objects
  yield
  after = ObjectSpace.count_objects
  after.each { |k,v| after[k] = v - before[k] }
  after[:T_HASH] -= 1 # probe effect - we created the before hash.
  GC.enable
  result = after.reject { |k,v| v == 0 }
  GC.start
  result
end

def master_version(key)
  Array({}.delete(:default)).compact
end

def fast_version(key)
  if key
    Array({}.delete(:default)).compact
  else
    []
  end
end

def test
  puts "master_version with false"
  puts allocate_count { 1000.times { master_version(false) } }
  puts "master_version with true"
  puts allocate_count { 1000.times { master_version(true) } }
  puts "fast_version with false"
  puts allocate_count { 1000.times { fast_version(false) } }
  puts "fast_version with true"
  puts allocate_count { 1000.times { fast_version(true) } }

  Benchmark.ips do |x|
    x.report("master_version with false")  { master_version(false) }
    x.report("master_version with true") { master_version(true) }
    x.report("fast_version with false")    { fast_version(false) }
    x.report("fast_version with true")   { fast_version(true) }
    x.compare!
  end
end

test
```

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