plugin: 'findFiles' should search ignored files when exclude is 'null'

[vince-fugnitto]

Bug Description:

Originally Reported: #8427.

The findFiles plugin API should search within ignored files when exclude: null according to vscode's documentation.
At the moment, searching with null yields no results when exclude is null (incompatibility with vscode).

Steps to Reproduce:

1. use the vscode extension: vscode-find-files
   (the extension searches for all readme files and prints those within node_modules (gitignored))
2. start the application
3. execute the command echo files
4. no results are found, while in vscode readme files are printed)