Fix `RuboCop::AST::NumericNode#sign?` to return boolean #1373

Job	Run time
Prism	27s
Coding Style	14s
Main Gem Specs \| RuboCop: master \| 3.4 \| parser_whitequark	1m 52s
Main Gem Specs \| RuboCop: master \| 3.4 \| parser_prism	2m 3s
Main Gem Specs \| RuboCop: master \| 2.7 \| parser_whitequark	2m 16s
Main Gem Specs \| RuboCop: v1.72.1 \| 2.7 \| parser_whitequark	2m 4s
Specs \| 3.3	12s
Specs \| 3.0	35s
Specs \| 3.2	10s
Specs "modern" \| 3.0	33s
Specs \| 3.4	11s
Specs \| jruby	43s
Coverage \| 2.7	28s
Specs \| jruby-head	41s
Specs \| truffleruby	46s
Specs \| head	11s
Specs \| 3.1	30s
Specs \| truffleruby-head	55s
Specs \| 2.7	29s
	15m 20s

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