Allow optional backend in Job

[ikkoham]

Summary

It is proposed that backend be set to None in the Job to use the Job that is not related to backend.
#8382 (comment)

Can we extend the job like this PR?
Or, should we design the specific job class?

Details and comments

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[coveralls]

Pull Request Test Coverage Report for Build 2754150868

• 5 of 7 (71.43%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.002%) to 83.991%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
qiskit/providers/job.py	5	7	71.43%

Files with Coverage Reduction	New Missed Lines	%
qiskit/providers/job.py	1	57.14%

Totals
Change from base Build 2748430390:	-0.002%
Covered Lines:	55898
Relevant Lines:	66552

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💛 - Coveralls

[ikkoham]

Closable since #8382 is merged.