polkadot: parachains: Fix v9 host configuration migration (#2103)

We shouldn't override with their default fields that have been added in
the previous version(v8), because we are going to lose whatever values have
been set.

Note, v8 & v9 seems to have landed at the same time on Rococo, probably
they will land at the same time on westend and other chains, so functionally
doesn't make much difference, but let's have this fixed for people that copy-paste
:D, like me.

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Signed-off-by: Alexandru Gheorghe <[email protected]>