Enable "sketchy-null-mixed" Flow check

.flowconfig

@@ -13,7 +13,7 @@
 sketchy-null-bool=off
 sketchy-null-string=off
 sketchy-null-number=error
-sketchy-null-mixed=off
+sketchy-null-mixed=error
 sketchy-number=error
 untyped-type-import=error
 nonstrict-import=off

src/error/GraphQLError.js

@@ -138,7 +138,13 @@ export function GraphQLError( // eslint-disable-line no-redeclare
     }, []);
   }
 
-  const _extensions = extensions || (originalError && originalError.extensions);
+  let _extensions = extensions;
+  if (_extensions == null && originalError != null) {
+    const originalExtensions = originalError.extensions;
+    if (originalExtensions != null && typeof originalExtensions === 'object') {
+      _extensions = originalExtensions;
+    }
+  }
 
   Object.defineProperties(this, {
     message: {

src/jsutils/inspect.js

@@ -41,7 +41,7 @@ function formatObjectValue(value, previouslySeenValues) {
   if (value) {
     const customInspectFn = getCustomFn(value);
 
-    if (customInspectFn) {
+    if (customInspectFn !== undefined) {
       // $FlowFixMe(>=0.90.0)
       const customValue = customInspectFn.call(value);
 

src/jsutils/invariant.js

@@ -8,8 +8,9 @@
  */
 
 export default function invariant(condition: mixed, message: string) {
+  const booleanCondition = Boolean(condition);
   /* istanbul ignore else */
-  if (!condition) {
+  if (!booleanCondition) {
     throw new Error(message);
   }
 }

src/language/__tests__/toJSONDeep.js

@@ -12,7 +12,7 @@
  * on any nested value which defines it.
  */
 export default function toJSONDeep<T>(value: T): T {
-  if (!value || typeof value !== 'object') {
+  if (value == null || typeof value !== 'object') {
     return value;
   }
 

src/language/__tests__/visitor-test.js

@@ -472,7 +472,7 @@ describe('Visitor', () => {
           'enter',
           node.kind,
           key,
-          parent && parent.kind ? parent.kind : undefined,
+          parent && parent.kind != null ? parent.kind : undefined,
         ]);
 
         checkVisitorFnArgs(ast, arguments);
@@ -484,7 +484,7 @@ describe('Visitor', () => {
           'leave',
           node.kind,
           key,
-          parent && parent.kind ? parent.kind : undefined,
+          parent && parent.kind != null ? parent.kind : undefined,
         ]);
 
         expect(argsStack.pop()).to.deep.equal([...arguments]);