Accommodate next execution time being unknown

A task's next execution time is unknown if, for example, it's
currently running. When it's unknown the nextExecution.time will be
missing from the json describing the task. This commit updates the
documentation test to accommodate this possibility.

Closes gh-42351

Author: wilkinsona

spring-boot-project/spring-boot-actuator-autoconfigure/src/test/java/org/springframework/boot/actuate/autoconfigure/endpoint/web/documentation/ScheduledTasksEndpointDocumentationTests.java

@@ -63,18 +63,15 @@ void scheduledTasks() {
 							fieldWithPath("cron.[].expression").description("Cron expression."),
 							fieldWithPath("fixedDelay").description("Fixed delay tasks, if any."),
 							targetFieldWithPrefix("fixedDelay.[]."), initialDelayWithPrefix("fixedDelay.[]."),
-							nextExecutionWithPrefix("fixedDelay.[].")
-								.description("Time of the next scheduled execution."),
+							nextExecutionWithPrefix("fixedDelay.[]."),
 							fieldWithPath("fixedDelay.[].interval")
 								.description("Interval, in milliseconds, between the end of the last"
 										+ " execution and the start of the next."),
 							fieldWithPath("fixedRate").description("Fixed rate tasks, if any."),
 							targetFieldWithPrefix("fixedRate.[]."),
 							fieldWithPath("fixedRate.[].interval")
 								.description("Interval, in milliseconds, between the start of each execution."),
-							initialDelayWithPrefix("fixedRate.[]."),
-							nextExecutionWithPrefix("fixedRate.[].")
-								.description("Time of the next scheduled execution."),
+							initialDelayWithPrefix("fixedRate.[]."), nextExecutionWithPrefix("fixedRate.[]."),
 							fieldWithPath("custom").description("Tasks with custom triggers, if any."),
 							targetFieldWithPrefix("custom.[]."),
 							fieldWithPath("custom.[].trigger").description("Trigger for the task."))
@@ -93,7 +90,10 @@ private FieldDescriptor initialDelayWithPrefix(String prefix) {
 	}
 
 	private FieldDescriptor nextExecutionWithPrefix(String prefix) {
-		return fieldWithPath(prefix + "nextExecution.time").description("Time of the next scheduled execution.");
+		return fieldWithPath(prefix + "nextExecution.time")
+			.description("Time of the next scheduled execution, if known.")
+			.type(JsonFieldType.STRING)
+			.optional();
 	}
 
 	private FieldDescriptor[] lastExecution() {