check if value is null for utilization graph

[cloudbehl]

@rawagner @suomiy sometime value provided by prometheus is null on the particular timestamp. So if that's the case the storage dashboard is breaking.

[coveralls]

Pull Request Test Coverage Report for Build 1517

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.005%) to 87.354%

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Totals
Change from base Build 1512:	0.005%
Covered Lines:	3426
Relevant Lines:	3759

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💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 1357

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.004%) to 87.145%

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Totals
Change from base Build 1581:	0.004%
Covered Lines:	3661
Relevant Lines:	4021

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💛 - Coveralls