Runtime check that project is a string

In a previous commit, I removed `.toString()` without validating what
the Typescript compiler would say. This change does a runtime check
that the project variable is an actual string.

src/servicesHost.ts

@@ -1,5 +1,6 @@
 import * as path from 'path';
 import type * as typescript from 'typescript';
+import { isString } from 'util';
 import * as webpack from 'webpack';
 import { getParsedCommandLine } from './config';
 import * as constants from './constants';
@@ -799,7 +800,7 @@ export function makeSolutionBuilderHost(
         // The `configFilePath` is the same value that is used as the `project` parameter of
         // `getCustomtransformers` below.
         const project = options.configFilePath;
-        if (project) {
+        if (typeof project === "string") {
           // Custom transformers need a reference to the `typescript.Program`, that reference is
           // unavailable during the the `getCustomTransformers` callback below.
           const transformers = getCustomTransformers(instance.loaderOptions, result.getProgram(), result.getProgram);