Allow zero sized unions to be defined

A union may be defined in C that contains only zero sized types. We
should in turn generate a rust union in this case.

Author: Dan Robertson

bindgen-tests/tests/expectations/tests/struct_with_anon_union_of_array_members.rs

@@ -0,0 +1,13 @@
+// bindgen-flags: --default-non-copy-union-style manually_drop
+struct bpf_array {
+  union {
+    struct {
+      struct {} __empty_value1;
+      char value1[];
+    };
+    struct {
+      struct {} __empty_value2;
+      char value2[];
+    };
+  };
+};

bindgen-tests/tests/headers/struct_with_anon_union_of_array_members.h

@@ -1801,7 +1801,9 @@ impl CompInfo {
             return (false, false);
         }
 
-        if layout.is_some_and(|l| l.size == 0) {
+        if layout.is_some_and(|l| l.size == 0) &&
+            union_style != NonCopyUnionStyle::ManuallyDrop
+        {
             return (false, false);
         }