Fix FlowBuilder.next().end() infinite loop

[injae-kim]

related issue #4432

Motivation:

builder.next(createCompleteStep("stepA"))
       .end()
       .start();
--
[main] INFO org.springframework.batch.core.job.SimpleStepHandler - Executing step: [stepA]
[main] INFO org.springframework.batch.core.job.SimpleStepHandler - Executing step: [stepA]
[main] INFO org.springframework.batch.core.job.SimpleStepHandler - Executing step: [stepA]
... "stepA" never finishes

• On Starting a flow with Flow#next makes the first step execute twice #4432, we found that FlowBuilder.next().end() never finishes
• Problem is that addTransition("COMPLETED", next); on next() although there's no step registered yet

Modification:

• If there's no step registered yet on FlowBuilder.next().end(), handle same as FlowBuilder.start().end() to avoid infinite loop

Result:

• FlowBuilder.next().end() works same as FlowBuilder.start().end() and finishes well
• Close Starting a flow with Flow#next makes the first step execute twice #4432 issue

[injae-kim]

		if (this.currentState == null) {
			doStart(input); // 1. currentState: input
		}

		State next = createState(input);
        // 2. input -> COMPLETED -> input -> COMPLETED -> input ..
		addTransition("COMPLETED", next); 

when currentState == null, I think we should not addTransition("COMPLETED", next); cause it makes infinite loop 🤔

[injae-kim]

When currrent == null, previous doFrom logic seems createState(input) twice (inside of doStart() and L273) so I fix it~!

[injae-kim]

Gentle ping to @fmbenhassine , can you review this PR? I need to fix this for my service🙇 thanks for your help!

[fmbenhassine]

LGTM 👍 Rebased and merged as 4a67b22. Thank you for your contribution!

[injae-kim]

Thanks a lot for review&merge! I'll make another contribution soon 🙇