populate map.sources and map.sourcesContent even if none of the original code is used

src/generators/Generator.js

@@ -149,6 +149,15 @@ export default class Generator {
 
 		const { filename } = options;
 
+		// special case — the source file doesn't actually get used anywhere. we need
+		// to add an empty file to populate map.sources and map.sourcesContent
+		if ( !parts.length ) {
+			compiled.addSource({
+				filename,
+				content: new MagicString( this.source ).remove( 0, this.source.length )
+			});
+		}
+
 		parts.forEach( str => {
 			const chunk = str.replace( pattern, '' );
 			if ( chunk ) addString( chunk );

test/sourcemaps/static-no-script/input.html

@@ -0,0 +1 @@
+<p>no moving parts</p>

test/sourcemaps/static-no-script/test.js

@@ -0,0 +1,9 @@
+const fs = require( 'fs' );
+const path = require( 'path' );
+
+export function test ({ assert, map }) {
+	assert.deepEqual( map.sources, [ 'input.html' ]);
+	assert.deepEqual( map.sourcesContent, [
+		fs.readFileSync( path.join( __dirname, 'input.html' ), 'utf-8' )
+	]);
+}