DAOS-16407 cart: coverity 2555825 fix

[frostedcmos]

• Added explicit assert check for provider_str not being null and removed a conditional check which confused coverity.

Required-githooks: true

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[github-actions]

Ticket title is '[coverity] 2555825 Dereference after null check'
Status is 'In Review'
Labels: 'SDLe,coverity'
https://daosio.atlassian.net/browse/DAOS-16407