NullPointerException calling getStartFormKey for multi-start event processes

Environment (Required on creation):

Camunda 7.17.0, all distros

Description (Required on creation; please attach any relevant screenshots, stacktraces, log files, etc. to the ticket):

Calling `getFormService().getStartFormKey(processDefinitionId)` for a process definition with multiple start events leads to (on JDK 17):

Caused by: java.lang.NullPointerException: Cannot invoke "org.camunda.bpm.engine.impl.form.FormDefinition.getFormKey()" because "formDefinition" is null
    at org.camunda.bpm.engine.impl.cmd.GetFormKeyCmd.execute(GetFormKeyCmd.java:84)
    at org.camunda.bpm.engine.impl.cmd.GetFormKeyCmd.execute(GetFormKeyCmd.java:40)
    at org.camunda.bpm.engine.impl.interceptor.CommandExecutorImpl.execute(CommandExecutorImpl.java:28)
    at org.camunda.bpm.engine.impl.interceptor.CommandContextInterceptor.execute(CommandContextInterceptor.java:110)
    at org.camunda.bpm.engine.impl.interceptor.ProcessApplicationContextInterceptor.execute(ProcessApplicationContextInterceptor.java:70)
    at org.camunda.bpm.engine.impl.interceptor.CommandCounterInterceptor.execute(CommandCounterInterceptor.java:35)
    at org.camunda.bpm.engine.impl.interceptor.LogInterceptor.execute(LogInterceptor.java:33)
    at org.camunda.bpm.engine.impl.FormServiceImpl.getStartFormKey(FormServiceImpl.java:99)

    ... user code

Steps to reproduce (Required on creation):

1. Deploy a process definition with multiple start events
2. Call getFormService().getStartFormKey(processDefinitionId) for that process definition

Observed Behavior (Required on creation):

NPE as described above

Expected behavior (Required on creation):

A meaningful NullValueException as thrown in GetStartFormCmd for example in the same setting

Root Cause (Required on prioritization):

• In GetFormKeyCmd lines 83 and 84, we expect there always to be a FormDefinition, which is not the case in 7.17.0 given the preconditions above.

  FormDefinition formDefinition = processDefinition.getStartFormDefinition();
  formKeyExpression = formDefinition.getFormKey();
  

Solution Ideas (Optional):

1. Throw a NullValueException stating the absence of a form definition for the start event

Hints (optional):

• The "get form key" behavior changed with regards to 7.16, where you would receive "null" instead of an NPE
• Other form data related methods/commands throw NullValueExceptions in such setups