fix: add missing checks for not nullptr in rpc for dash specific code

[knst]

Issue being fixed or feature implemented

There're some usages of unique pointers in RPC code that are not guarded by non-nullptr checks

What was done?

Added missing CHECK_NON_FATAL and refactored some of them to EnsureConnman and EnsurePeerman

How Has This Been Tested?

Run unit/functional tests

Breaking Changes

N/A

Checklist:

• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added or updated relevant unit/integration/functional/e2e tests
• I have made corresponding changes to the documentation
• I have assigned this pull request to a milestone

[UdjinM6]

utACK

[PastaPastaPasta]

utACK dc15420