Partly fix "sketchy-null-bool" Flow rule (#1881)

Author: IvanGoncharov

src/execution/values.js

@@ -146,11 +146,12 @@ export function getArgumentValues(
     let isNull;
     if (argumentNode && argumentNode.value.kind === Kind.VARIABLE) {
       const variableName = argumentNode.value.name.value;
-      hasValue = variableValues && hasOwnProperty(variableValues, variableName);
-      isNull = variableValues && variableValues[variableName] === null;
+      hasValue =
+        variableValues != null && hasOwnProperty(variableValues, variableName);
+      isNull = variableValues != null && variableValues[variableName] === null;
     } else {
       hasValue = argumentNode != null;
-      isNull = argumentNode && argumentNode.value.kind === Kind.NULL;
+      isNull = argumentNode != null && argumentNode.value.kind === Kind.NULL;
     }
 
     if (!hasValue && argDef.defaultValue !== undefined) {

src/language/blockString.js

@@ -80,7 +80,7 @@ function isBlank(str) {
 export function printBlockString(
   value: string,
   indentation?: string = '',
-  preferMultipleLines?: ?boolean = false,
+  preferMultipleLines?: boolean = false,
 ): string {
   const isSingleLine = value.indexOf('\n') === -1;
   const hasLeadingSpace = value[0] === ' ' || value[0] === '\t';

src/validation/rules/VariablesInAllowedPosition.js

@@ -97,7 +97,7 @@ function allowedVariableUsage(
 ): boolean {
   if (isNonNullType(locationType) && !isNonNullType(varType)) {
     const hasNonNullVariableDefaultValue =
-      varDefaultValue && varDefaultValue.kind !== Kind.NULL;
+      varDefaultValue != null && varDefaultValue.kind !== Kind.NULL;
     const hasLocationDefaultValue = locationDefaultValue !== undefined;
     if (!hasNonNullVariableDefaultValue && !hasLocationDefaultValue) {
       return false;