fix potential null deref in `merge_vararg_unions` (#59525)
so as to avoid segfaults in the following type normalization
```
julia> Union{
Tuple{},
Tuple{Int},
Tuple{UInt},
Tuple{UInt, Vararg{UInt}},
Tuple{Int, Int, Vararg{Int}}
};
[89686] signal 11 (2): Segmentation fault: 11
in expression starting at REPL[2]:1
jl_is_tuple_type at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-XG3Q6T6R70.0/build/default-honeycrisp-XG3Q6T6R70-0/julialang/julia-master/src/./julia.h:1835 [inlined]
merge_vararg_unions at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-XG3Q6T6R70.0/build/default-honeycrisp-XG3Q6T6R70-0/julialang/julia-master/src/jltypes.c:617
ijl_type_union at /Users/julia/.julia/scratchspaces/a66863c6-20e8-4ff4-8a62-49f30b1f605e/agent-cache/default-honeycrisp-XG3Q6T6R70.0/build/default-honeycrisp-XG3Q6T6R70-0/julialang/julia-master/src/jltypes.c:674
jl_apply at /Users/julia/.julia/scratchspaces/a66863
```
