Allow empty requirements.txt

[craigkerstiens]

Viewing requirements.txt as a canonical form for specifying dependencies it should allow for an empty requirements.txt to be installed in case a package needs no other dependencies.

Currently it throws the exception:
You must give at least one requirement to install (see "pip help install")

This does not need to be the case for any pip install, but should be allowed when a requirements file is specified.

[jwineinger]

+1

[mihneadb]

#580

[xavfernandez]

Since pip 1.1, it simply prints a warning but does not error anymore:

$ touch empty.txt
$ pip install -r empty.txt
You must give at least one requirement to install (see "pip help install")
$ echo $?
0

[dstufft]

This seems to be the case now.