fix(eslint-plugin): [no-unnecessary-condition] account for optional chaining on potentially void values (#6432)

* Update error message per #5255 discussion

* Add void check to no-unnecessary-condition and test case

* Cleanup, update type checks

Author: cparros

packages/eslint-plugin/src/rules/no-unnecessary-condition.ts

@@ -574,10 +574,11 @@ export default createRule<Options, MessageId>({
         node.type === AST_NODE_TYPES.MemberExpression
           ? !isNullableOriginFromPrev(node)
           : true;
+      const possiblyVoid = isTypeFlagSet(type, ts.TypeFlags.Void);
       return (
-        isTypeAnyType(type) ||
-        isTypeUnknownType(type) ||
-        (isNullableType(type, { allowUndefined: true }) && isOwnNullable)
+        isTypeFlagSet(type, ts.TypeFlags.Any | ts.TypeFlags.Unknown) ||
+        (isOwnNullable &&
+          (isNullableType(type, { allowUndefined: true }) || possiblyVoid))
       );
     }
 

packages/eslint-plugin/tests/rules/no-unnecessary-condition.test.ts

@@ -346,6 +346,10 @@ do {} while (true);
       options: [{ allowConstantLoopConditions: true }],
     },
     `
+let variable = 'abc' as string | void;
+variable?.[0];
+    `,
+    `
 let foo: undefined | { bar: true };
 foo?.bar;
     `,