fix: some directories weren't searched in JDK21

The value of `File.length()` is unspecified for directories.

Author: slisson

build-tools-lib/src/main/kotlin/org/modelix/buildtools/ModulesMiner.kt

@@ -62,7 +62,7 @@ class ModulesMiner() {
 
         if (isIgnored(file, fileFilter)) return
 
-        if (file.length() == 0L) return
+        if (file.isFile && file.length() == 0L) return
 
         if (file.isFile) {
             when (file.extension.lowercase()) {