Fix conversion routines

[andrewstucki]

One of the cloud tests picked up an issue with our conversion code:

constructing desired pools: json: Unmarshal(nil *v1.ResourceRequirementsApplyConfiguration)

This is pointing to the second argument in our convertJSON* routines being passed as a nil. This PR adds nil guards/initializations to the target structs where necessary.

[RafalKorepta]

LGTM, It would be good to add regression tests.

[chrisseto]

Agreed with Rafal, a regression test would be great!

[andrewstucki]

Done, just added a resources block to one of the golden file cases in our lifecycle tests. Without the change we get:

        	Error Trace:	/Users/andrewstucki/code/redpanda-operator/operator/internal/lifecycle/v2_test.go:158
        	Error:      	Received unexpected error:
        	            	json: Unmarshal(nil *v1.ResourceRequirementsApplyConfiguration)
        	Test:       	TestV2ResourceClient/compat-test

With it, the tests pass.