Fix handling of sectioned source maps missing 'names' array

AFAIK just like for regular source maps, the 'names' array can be
omitted for maps in sectioned/index source maps.

Currently, trace-mapping will throw with an exception
"Cannot read properties of undefined (reading 'length')"
in case no 'names' array is provided.

src/any-map.ts

@@ -111,7 +111,9 @@ function addSection(
   const { resolvedSources, sourcesContent: contents } = map;
 
   append(sources, resolvedSources);
-  append(names, map.names);
+  if (map.names) {
+    append(names, map.names);
+  }
   if (contents) append(sourcesContent, contents);
   else for (let i = 0; i < resolvedSources.length; i++) sourcesContent.push(null);
 

test/any-map.test.ts

@@ -58,7 +58,6 @@ describe('AnyMap', () => {
               offset: { line: 0, column: 1 },
               map: {
                 version: 3,
-                names: [],
                 sources: ['fourth.js'],
                 sourcesContent: ['fourthsource'],
                 mappings: 'AAAA',