[vm] Relax assertion for empty record type
[vm] Relax assertion for empty record type

TEST=co19/LanguageFeatures/Records/upper_and_lower_bounds_A04_t01
(Tested manualy by applying 4b6a8f35b940f77e0d892537bf3029a61ae53e20 and running
"tools/test.py -n dartk-strong-linux-debug-x64 co19/LanguageFeatures/Records/upper_and_lower_bounds_A04_t01").

Issue: https://github.com/dart-lang/sdk/issues/49719
Change-Id: Ib5658ac776a75913e617095f8861baba25f13de3
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/261800
Reviewed-by: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Commit-Queue: Ryan Macnak <d738b7f001baf9789095d7dc549cff6de60b011d@google.com>
Auto-Submit: Alexander Markov <239e774fd29abc1267878f57afe1a8a03e0ca887@google.com>

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