refactor(audioconv): Improve option handling and flexibility

- Updated `resolveOptions` to return `defaultOptions` when the input is an empty object or not an object.
- Set the default value of `useDefault` parameter in `resolveOptions` to false.
- Refined `convertAudio` function to apply `ffmpeg` audio options only when specified, allowing for more flexible conversion with minimal configurations.

lib/audioconv.js

@@ -39,7 +39,8 @@ const {
   createDirIfNotExistSync,
   createLogFile,
   dropNullAndUndefined,
-  isNullOrUndefined
+  isNullOrUndefined,
+  isObject
 } = require('./utils');
 
 /**
@@ -193,7 +194,9 @@ function splitOptions(options) {
  * @return {module:audioconv~ResolvedConvertAudioOptions} The resolved options.
  * @since  1.0.0
  */
-function resolveOptions(options, useDefault=true) {
+function resolveOptions(options, useDefault=false) {
+  if (!isObject(options)) return defaultOptions;
+
   return {
     inputOptions: (
       Array.isArray(options?.inputOptions)
@@ -457,12 +460,18 @@ async function convertAudio(inFile, options = defaultOptions) {
           || (Array.isArray(options.outputOptions) && options.outputOptions.length === 0)
       )
     ) {
-      ffmpegChain
-        .audioBitrate(options.bitrate)
-        .audioCodec(options.codec)
-        .audioChannels(options.channels)
-        .audioFrequency(options.frequency)
-        .outputFormat(options.format);
+      // Add each option only if present and specified
+      // By using this logic, now user can convert an audio file with only specifying
+      // the output audio format
+      if (options.bitrate) ffmpegChain.audioBitrate(options.bitrate);
+      if (options.codec) ffmpegChain.audioCodec(options.codec);
+      if (options.channels) ffmpegChain.audioChannels(options.channels);
+      if (options.frequency) ffmpegChain.audioFrequency(options.frequency);
+
+      // Mandatory for output format
+      // Note: Specifying the same output format as input may copy the input file
+      //       with the same codec and format
+      ffmpegChain.outputFormat(options.format);
     } else {
       if (Array.isArray(options.inputOptions)) {
         ffmpegChain.inputOptions(options.inputOptions);