return nil for undefined commands

[velimir]

revert behaviour introduced in /pull/1255

Seems like it broke compile command for all projects w/o :compilation-dir defined. At least for me on attempt to compile project, command fails with a message:

user-error: The value for: compilation-dir in project-type: rebar was neither a function nor a string.

projectile-compilation-dir function has a fallback to projectile-project-root as a default compilation directory, but new behaviour of projectile-default-generic-command throws an error on attempt to a get command-type from a hash, which is not defined for some project type (most of them).

[velimir]

/bin/sh: 1: cask: not found
cask install
make: cask: Command not found
make: *** [elpa] Error 127

🤔

[bbatsov]

Closing if favour of #1269.