Allow array as value in externals object.

Fixes https://github.com/webpack/webpack/issues/8041.

schemas/WebpackOptions.json

@@ -123,6 +123,9 @@
               {
                 "type": "object"
               },
+              {
+                "$ref": "#/definitions/common.arrayOfStringValues"
+              },
               {
                 "type": "boolean"
               }

test/configCases/externals/optional-externals-array/index.js

@@ -0,0 +1,10 @@
+it("should not fail on optional externals", function() {
+	try {
+		require("external");
+	} catch(e) {
+		expect(e).toBeInstanceOf(Error);
+		expect(e.code).toBe("MODULE_NOT_FOUND");
+		return;
+	}
+	throw new Error("It doesn't fail");
+});

test/configCases/externals/optional-externals-array/webpack.config.js

@@ -0,0 +1,5 @@
+module.exports = {
+	externals: {
+		external: ["./math", "subtract"]
+	}
+};