Handle dynamic mocks specially for null-object doubles #805

Job	Run time
Rubocop	29s
Ruby 3.3	33s
Ruby 2.7 (Windows)	5m 2s
Legacy Ruby Builds (2.1.9)	37s
Ruby 3.2	24s
Legacy Ruby Builds (2.0)	35s
Ruby 2.6 (Windows)	3m 30s
Legacy Ruby Builds (1.9.3)	44s
Ruby 2.5 (Windows)	3m 43s
Ruby 3.1	27s
Legacy Ruby Builds (1.9.2)	44s
Legacy Ruby Builds (1.8.7)	38s
Ruby 2.4 (Windows)	2m 53s
Ruby 3.0	26s
Ruby 2.3 (Windows)	2m 29s
Legacy Ruby Builds (REE)	51s
Ruby 2.2 (Windows)	2m 7s
Legacy Ruby Builds (JRuby 1.7)	42s
Ruby 2.7	48s
Legacy Ruby Builds (JRuby 1.7 1.8 mode)	40s
Ruby 2.6	52s
Legacy Ruby Builds (JRuby 9.1.17.0)	46s
Ruby 2.5	1m 0s
Ruby 2.4	48s
Ruby 2.3	44s
Ruby 2.2	1m 11s
Ruby ruby-head	30s
Ruby jruby-9.2.13.0	1m 42s
Ruby 2.7 with diff-lcs 1.3	44s
Ruby 2.7 with diff-lcs 1.4.3	49s
	37m 28s

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