Fix nil pathname

lib/slather/project.rb

@@ -185,7 +185,15 @@ def create_coverage_files(binary_path, path_objects)
         if paths_to_segments.key?(coverage_file.source_file_pathname)
           coverage_file.segments = paths_to_segments[coverage_file.source_file_pathname]
         end
-        !coverage_file.ignored? && coverage_file.include_file? ? coverage_file : nil
+        puts "--> coverage_file.source_file_pathname: #{coverage_file.source_file_pathname}"
+
+        if coverage_file.source_file_pathname.nil? || coverage_file.source_file_pathname.empty?
+          puts "--> source_file_pathname is nil or empty, source: #{source}"
+        else
+          puts "--> source_file_pathname is not nil or empty"
+        end
+
+        coverage_file.source_file_pathname && !coverage_file.ignored? && coverage_file.include_file? ? coverage_file : nil
       end.compact
     end
     private :create_coverage_files
@@ -651,7 +659,7 @@ def matches_arch(binary_path)
 
     def find_source_files
       source_files = load_option_array("source_files")
-      return if source_files.nil?
+      return [] if source_files.nil?
 
       current_dir = Pathname("./").realpath
       paths = source_files.flat_map { |pattern| Dir.glob(pattern) }.uniq