fix: respect inputSourceMap loader option (#896)

Currently, when the loader is invoked without a `map` object the `inputSourceMap` fallbacks to `undefined`.

This caused the sourcemap to be loaded by Babel since this defaults to `true` https://babeljs.io/docs/en/options#inputsourcemap

Thus, currently there is no way to disable `inputSourceMap` when using this loader.

src/index.js

@@ -130,7 +130,7 @@ async function loader(source, inputSourceMap, overrides) {
 
   const programmaticOptions = Object.assign({}, loaderOptions, {
     filename,
-    inputSourceMap: inputSourceMap || undefined,
+    inputSourceMap: inputSourceMap || loaderOptions.inputSourceMap,
 
     // Set the default sourcemap behavior based on Webpack's mapping flag,
     // but allow users to override if they want.