AST reimport fails for inline assembly with uninitialized variable declarations

[cameel]

An attempt to import an AST produced by the compiler fails when the original source code contains an inline assembly block with a variable declaration without an initializer:

test.sol

contract C {
    function f() public pure {
        assembly {
            let x
        }
    }
}

solc test.sol --combined-json ast | solc --import-ast -

Failed to import AST: Expected "nodeType" to be of type string!

This is reproducible all the way down to 0.6.2, when the --import-ast option was introduced, though before 0.8.0 the error message was different and it seems to be failing for a different reason (even examples that work on 0.8.x fail on those versions)

Failed to import AST: Top-level node should be a 'SourceUnit'

Discovered by @aart in #13576 (comment).