Use automatic type conversion to convert from () and T into T with Merge nodes

This is an idea that will require some discussion to determine the feasibility and approach before implementation may begin.

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When a Merge node receives only one input (either only the bottom input or only the left input), it can get stuck with some other random input type for the other, which doesn't match and results in a type error. This is only subtly visible because the two input connectors don't have the same color, but that is hard to spot.

Instead, we want the Merge node to receive the `()` type when it's not connected to anything, and use the automatic type conversion system to convert from `()` to `T`, for example `()` to `List` if it needs to match with the other input of type `List`.

首先追踪 Merge 节点的实现和自动类型转换系统，以了解当前如何为未连接的输入确定类型。确定是否可以将缺失的输入表示为 () 并将其转换为另一个输入的类型，然后在开始实现之前记录实现方案和所需的测试。

由索引模型根据 Issue 内容生成。