[analyzer] Model initialization of const global variables without initialization expression in C

As @earnol discovered during discussing #199271, the following C code is not modelled properly by ClangSA.

```c
const int a[4] = {};
const int b[4];

int test() {
clang_analyzer_eval(a[0] == 0) // TRUE
clang_analyzer_eval(b[0] == 0) // should be TRUE, but currently is UNKNOWN
return 0;
}
```

In C, it is valid to have a const global variable without an initializer expression, and according to the language spec, it should be initialized to 0.