fix: prevent crash when listing symbols in invalid config

[radeksimko]

We previously ignored all diagnostics, which includes both minor and major, the latter of which prevent the HCL parser from parsing anything and therefore resulting in nil being returned from ParseBlockFromTokens which was then unexpected outcome for Blocks().

Example of such invalid config

provider "aws"
  access_key = "value"
}

The crash didn't occur during completion because it uses some other position-based methods which would already stop before stepping further into the parser internals and in particular into ParseBlockFromTokens. Symbol listing however is not position based and so listing symbols on an invalid config leads to a crash.

[ghost]

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