Fix 20934 - preview=dtorfields segfaults for disabled/extern construc…

[MoonlightSentinel]

…tors

Don't try to insert a destructor body if there's no body because
the following semantic would segfault due to the missing try-block

Extracted from #11656

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	20934	normal	preview=dtorfields segfaults for disabled/extern constructors

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + dmd#11658"