`Builder?` causes invalid FunC code — `store_slice` used instead of proper serialization

[Gusarich]

Using an optional Builder compiles in Tact but crashes during FunC compilation. The docs say all primitive types, including Builder, can be nullable — but the compiler generates invalid code when serializing Builder?.

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Example:

contract Test {
    builderOpt: Builder?;

    init() {
        self.builderOpt = null;
    }

    receive() {
        self.builderOpt = null;
    }
}

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Expected:
Compiles fine. Docs say:

"All primitive types, as well as Structs and Messages, can be nullable..."

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Actual:
FunC crash:

FunC compilation error: ...: error: cannot apply function store_slice : (builder, slice) -> builder to arguments of type (builder, builder): cannot unify type builder with slice
      build_0 = build_0.store_maybe_ref(begin_cell().store_slice(v'builderOpt).end_cell());

Compiler tries to store_slice a Builder, which makes no sense.

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Conclusion:
Type system allows Builder?, but codegen doesn't handle it — using store_slice for a builder.

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LLM Fuzzing discovery (see #2490)