Fix empty query when using --excludeExternalTargets with --useCquery

[GuillaumeVW]

Fixes an issue where using --excludeExternalTargets with --useCquery would cause bazel-diff to pass an empty query string to bazel query, resulting in a failure with exit code 2.
The problem was that when external targets are excluded, the code was still trying to execute a query on an empty list, which produced an empty string. Now we check if there are actually any repo targets to query before executing the query, otherwise we just skip it and use an empty list.

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