[fix] null guard for public key in PKey#toJava #335

Job	Run time
maven-test (21, oracle, jruby-9.3.13.0)	2m 30s
maven-test (11, zulu, jruby-9.4.5.0)	2m 51s
maven-test (jruby-9.4.14.0, 25, temurin)	2m 57s
maven-test (jruby-9.4.14.0, 11, temurin)	2m 51s
maven-test (11, temurin, jruby-9.2.20.1)	2m 53s
maven-test (11, temurin, jruby-9.3.13.0)	2m 33s
maven-test (8, temurin, jruby-9.3.3.0)	2m 45s
maven-test (jruby-9.4.14.0, 21, temurin)	2m 59s
maven-test (jruby-9.4.14.0, 8, temurin)	2m 47s
maven-test (8, temurin, jruby-9.2.19.0)	3m 25s
maven-test (21, temurin, jruby-10.0.4.0)	2m 23s
maven-test (21, oracle, jruby-9.4.8.0)	2m 40s
maven-test (24, zulu, jruby-10.0.1.0)	2m 47s
maven-test (25, temurin, jruby-10.0.3.0)	3m 15s
maven-test (21, corretto, jruby-head)	3m 1s
	42m 37s

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