fix segfault with gensym node instantiation

[metagn]

fixes #24048

Generic lambdas get instantiated via replaceTypesInBody which calls replaceTypeVarsN on the body of the lambda. This body can contain sym nodes of gensym symbols generated by macros, which have nil type. But a piece of code in replaceTypeVarsN checks whether the type of a symbol is equal to void without checking if it's nil first, which causes a segfault. Now it also checks that the type of the symbol isn't nil for it to be void.

[github-actions]

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The lines below are statistics of the Nim compiler built from d27061f

Hint: mm: orc; opt: speed; options: -d:release
174003 lines; 8.758s; 655.379MiB peakmem