Allow unset of entrypoint (resolves #5582)
Allow unset of entrypoint (resolves #5582)

When an empty string is passed to the 'entrypoint' parameter, for example
`docker-compose run --entrypoint='' ...`
OR
`docker-compose run --entrypoint '' ...`
It allows the default entrypoint to be overriden by empty value.

Signed-off-by: Ganesh Satpute <937215f824466c3a2a3a1f4b616eba701550f193@gmail.com>

shin-/compose,vdemeester/compose,vdemeester/compose,thaJeztah/compose,funkyfuture/docker-compose,dnephin/compose,thaJeztah/compose,shin-/compose,funkyfuture/docker-compose,dnephin/compose