translationtool crashes when there are no plural form

[StCyr]

when there are no plural form in an app, "translationtool.phar create-pot-file" doesn't add the "Plural-Forms: nplurals=..." header line.

That make the command "translationtool.phar convert-po-files" crash:

root@2408LC07E43:/var/www/nc29/apps/signlive# ./translationtool.phar convert-po-files
Those files are ignored:
Array
(
    [0] => js/
)
PHP Fatal error:  Uncaught TypeError: TranslatableApp::writeJsFile(): Argument #2 ($plurals) must be of type string, null given, called in phar:///var/www/nc29/apps/signlive/translationtool.phar/translationtool.php on line 179 and defined in phar:///var/www/nc29/apps/signlive/translationtool.phar/translationtool.php:184
Stack trace:
#0 phar:///var/www/nc29/apps/signlive/translationtool.phar/translationtool.php(179): TranslatableApp->writeJsFile()
#1 phar:///var/www/nc29/apps/signlive/translationtool.phar/translationtool.php(558): TranslatableApp->createNextcloudFiles()
#2 phar:///var/www/nc29/apps/signlive/translationtool.phar/translationtool.php(688): TranslationTool->convertPoFiles()
#3 /var/www/nc29/apps/signlive/translationtool.phar(12): require('...')
#4 {main}
  thrown in phar:///var/www/nc29/apps/signlive/translationtool.phar/translationtool.php on line 184

Steps to reproduce

1. Have an application without any plural form defined.
2. translationtool.phar create-pot-file
3. translationtool.phar convert-po-files