Empty profile directory causes launcher to pass malformed start arguments to Minecraft

Affected version: 2.0.757 (Windows)

The bug

The launcher passes malformed start arguments to Minecraft when you leave the profile directory empty. Such an empty profile directory value can be created using MCL-5440.

The passed arguments are invalid because Minecraft fails loading the assets and instead creates an folder called -assetsDir (which is the argument key for the assets folder name) in the %appdata%\.minecraft folder.

Additionally the launcher prints a malformed log:

[0123/125946:INFO:GameCallbacks.cpp(161)] game/game () error Completely ignored arguments: [C:/Users\###\AppData\Roaming\.minecraft/assets]
  <log4j:Event logger="bzn" timestamp="1485172786913" level="ERROR" thread="Client thread">
    <log4j:Message><![CDATA[Can't find the resource index file: --assetsDir\assets\indexes\1.11.json]]></log4j:Message>
  </log4j:Event>
[0123/125946:INFO:GameCallbacks.cpp(161)] launcher/monitor (Process Monitor) error expected <

And the Game Output window seems to handle that many log line very bad while they are being printed. Only updating when you click somewhere.

How to reproduce

Important: This creates malformed folders and might corrupt files in the %appdata%\.minecraft folder.

1. Use MCL-5440 to get a profile with an empty profile directory

2. Start Minecraft with this profile