Fix crash when icon_name is None

Fix crash when icon_name is None

Fixes the following crash:

```
$ fdroid update --create-metadata --rename-apks
WARNING: Using Java's jarsigner, not recommended for verifying APKs! Use apksigner
CRITICAL: Unknown exception found!
Traceback (most recent call last):
  File "/home/jonas/miniconda3/bin/fdroid", line 164, in <module>
    main()
  File "/home/jonas/miniconda3/bin/fdroid", line 138, in main
    mod.main()
  File "/home/jonas/miniconda/lib/python3.6/site-packages/fdroidserver/update.py", line 1927, in main
    apks, cachechanged = process_apks(apkcache, repodirs[0], knownapks, options.use_date_from_apk)
  File "/home/jonas/miniconda/lib/python3.6/site-packages/fdroidserver/update.py", line 1454, in process_apks
    use_date_from_apk, ada, True)
  File "/home/jonas/miniconda/lib/python3.6/site-packages/fdroidserver/update.py", line 1327, in process_apk
    apk = scan_apk(apkfile)
  File "/home/jonas/miniconda/lib/python3.6/site-packages/fdroidserver/update.py", line 1056, in scan_apk
    scan_apk_aapt(apk, apk_file)
  File "/home/jonas/miniconda/lib/python3.6/site-packages/fdroidserver/update.py", line 1184, in scan_apk_aapt
    apk['icons_src'] = _get_apk_icons_src(apkfile, icon_name)
  File "/home/jonas/miniconda/lib/python3.6/site-packages/fdroidserver/update.py", line 1089, in _get_apk_icons_src
    density_re = re.compile('^res/(.*)/' + icon_name + '\.(png|xml)$')
TypeError: must be str, not NoneType
```