Add warning when peer_id is not available when building topology (#2140)

... see https://github.com/paritytech/polkadot-sdk/issues/2138 for why
is not good, until we fix it let's add a warning to understand if this
is happening in the wild.

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Signed-off-by: Alexandru Gheorghe <alexandru.gheorghe@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>