Handle When.MAYBE warning

- Add com.google.code.findbugs:jsr305 to compileOnly
  order to get rid of warnings.

gradle.properties

@@ -6,3 +6,4 @@ st4Version=4.3.1
 jimfsVersion=1.2
 gradleEnterpriseVersion=3.10.3
 springGeConventionsVersion=0.0.11
+findbugsVersion=3.0.2

spring-shell-core/spring-shell-core.gradle

@@ -12,6 +12,7 @@ dependencies {
 	api('org.springframework:spring-messaging')
 	api('org.jline:jline')
 	api('org.antlr:ST4')
+	compileOnly 'com.google.code.findbugs:jsr305'
 	testImplementation 'org.springframework.boot:spring-boot-starter-test'
 	testImplementation 'org.awaitility:awaitility'
 	testImplementation 'com.google.jimfs:jimfs'

spring-shell-management/spring-shell-management.gradle

@@ -15,5 +15,6 @@ dependencies {
 		api "org.jline:jline-terminal-jna:$jlineVersion"
 		api "org.antlr:ST4:$st4Version"
 		api "com.google.jimfs:jimfs:$jimfsVersion"
+		api "com.google.code.findbugs:jsr305:$findbugsVersion"
 	}
 }

spring-shell-standard/spring-shell-standard.gradle

@@ -7,5 +7,6 @@ description = 'Spring Shell Standard'
 dependencies {
 	management platform(project(":spring-shell-management"))
 	implementation project(':spring-shell-core')
+	compileOnly 'com.google.code.findbugs:jsr305'
 	testImplementation 'org.springframework.boot:spring-boot-starter-test'
 }

spring-shell-table/spring-shell-table.gradle

@@ -8,5 +8,6 @@ dependencies {
 	management platform(project(":spring-shell-management"))
 	api('org.springframework:spring-core')
 	api('org.springframework:spring-context')
+	compileOnly 'com.google.code.findbugs:jsr305'
 	testImplementation 'org.springframework.boot:spring-boot-starter-test'
 }