test(bigtable): fix remaining JSpecify CI issues on Java 8 #23179

Job	Run time
bulk-filter	47s
changes	1m 17s
lint	3m 51s
windows	2m 18s
units (8)	56s
gapic-libraries-bom	20m 42s
enforcer	21m 4s
units (25)	37s
units (21)	36s
units (17)	36s
units (26)	39s
units (11)	33s
split-units (java-bigtable, 8)	6m 23s
split-clirr (java-bigtable)	2m 8s
split-units (java-bigtable, 17)	7m 52s
split-units (java-bigtable, 25)	7m 36s
split-units (java-bigtable, 26)	8m 26s
split-units (java-bigtable, 11)	6m 8s
split-units (java-bigtable, 21)	7m 35s
split-dependencies (java-bigtable)	5m 15s
conditional-required-check	4s
	1h 45m 23s

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