use src map if dst is nil and can't be set

[ndeloof]

As a struct field of type map is processed, and was not initialized as non-nil, don't try to set with a fresh new map and just adopt src value.
As I'm not sure about the potential side-effects, I've kept the original logic for cases where dst can be set.

fix #90

related:

• docker compose panics with volume override docker-archive/compose-cli#1261
• Docker panic when merging volume configs docker/cli#2916
• Stack deploy with volume overriding fails docker/cli#1981

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[ulyssessouza]

Hello @imdario!
Could you please have a look at this PR, so I can continue with compose-spec/compose-go#106 that depends on it.

Thanks!

[darccio]

@ulyssessouza Done. Thanks @ndeloof!