[INTERNAL] bundle/Builder: Correct bundling of resources with empty source map

If a resource features an empty source map (which is not the same as
having no source map), the bundle's source map generation needs to
handle this differently from cases where mappings exist.

It must not prefix the empty mapping with a string "AAAA,"
since the danglíng comma will result in a corrupt source map.

However, it should still add the mapping for the first line/column
"AAAA". Otherwise the source map of the preceding module in the bundle
would be applied.

Resolves the root cause of UI5/openui5#4035
DINC0139775

(cherry picked from commit 1228db7)

Author: RandomByte

lib/lbt/bundle/Builder.js

@@ -353,7 +353,13 @@ class BundleBuilder {
 			// If first column of the first line is not already mapped, add a mapping for the same reason as above.
 			// This is typical for transpiled code, where there is a bunch of generated code at the beginning that
 			// can't be mapped to the original source
-			map.mappings = "AAAA," + map.mappings;
+			if (map.mappings) {
+				map.mappings = "AAAA," + map.mappings;
+			} else {
+				// If there are no existing mappings (e.g. if the file is empty or only comments),
+				// make sure to still define a single mapping
+				map.mappings = "AAAA";
+			}
 		}
 
 		map.sourceRoot = path.posix.relative(

test/expected/build/application.a/dest-deps/resources/library/d/empty-dbg.js

@@ -0,0 +1,3 @@
+/*!
+ * Some fancy copyright
+ */

test/expected/build/application.a/dest-deps/resources/library/d/empty.js

@@ -0,0 +1,3 @@
+/*!
+ * Some fancy copyright
+ */