fix(federation/buildSubgraphSchema): do not create an invalid _Entity union if there is no entity defined

[ardatan]

Closes #5884

[changeset-bot]

🦋 Changeset detected

Latest commit: c1dfbe2

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 2 packages

Name	Type
@graphql-tools/federation	Patch
federation-benchmark	Patch

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🚀 Snapshot Release (alpha)

The latest changes of this PR are available as alpha on npm (based on the declared changesets):

Package	Version	Info
@graphql-tools/federation	1.1.15-alpha-20240208110858-c1dfbe2a7a411dbf6307a33ce330f7cfa0ace7d2	npm ↗︎ unpkg ↗︎

[github-actions]

✅ Benchmark Results

     ✓ no_errors
     ✓ expected_result

     checks.........................: 100.00% ✓ 336       ✗ 0  
     data_received..................: 39 MB   3.9 MB/s
     data_sent......................: 144 kB  14 kB/s
     http_req_blocked...............: avg=3.99µs   min=1.96µs   med=2.64µs  max=186.13µs p(90)=3.73µs   p(95)=4.24µs  
     http_req_connecting............: avg=782ns    min=0s       med=0s      max=131.39µs p(90)=0s       p(95)=0s      
     http_req_duration..............: avg=55.56ms  min=45.86ms  med=53.05ms max=143.1ms  p(90)=61.07ms  p(95)=87.02ms 
       { expected_response:true }...: avg=55.56ms  min=45.86ms  med=53.05ms max=143.1ms  p(90)=61.07ms  p(95)=87.02ms 
     http_req_failed................: 0.00%   ✓ 0         ✗ 168
     http_req_receiving.............: avg=135.73µs min=106.06µs med=127µs   max=471.78µs p(90)=150.51µs p(95)=179.66µs
     http_req_sending...............: avg=36.12µs  min=16.19µs  med=23.31µs max=1.94ms   p(90)=29.32µs  p(95)=33.91µs 
     http_req_tls_handshaking.......: avg=0s       min=0s       med=0s      max=0s       p(90)=0s       p(95)=0s      
     http_req_waiting...............: avg=55.39ms  min=45.71ms  med=52.9ms  max=142.76ms p(90)=60.91ms  p(95)=86.6ms  
     http_reqs......................: 168     16.726654/s
     iteration_duration.............: avg=59.76ms  min=49.34ms  med=56.84ms max=149.95ms p(90)=65.22ms  p(95)=90.8ms  
     iterations.....................: 168     16.726654/s
     vus............................: 1       min=1       max=1
     vus_max........................: 1       min=1       max=1

[github-actions]

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