scripts section: convert blank evironemnt variable as '' not consistent with run directly in bash

[gufengxiaoyuehan]

blank environment variable[''] behavior not associated with bash used as arguments in scripts

Issue description

when use pipenv script load envrionmnet variables as argument options , failed to use as optonial switch argument.

example script below:
pipenv version:

pipenv, version 2018.11.26

Pipfile

[[source]]
name = "pypi"
url = "https://pypi.org/simple"
verify_ssl = true

[dev-packages]

[packages]

[requires]
python_version = "3.6"
[scripts]
variable_args="./list_args.py ${switch_variable} ${position_variable}"

list_args.py

#!/usr/bin/env python
import sys
print(sys.argv)

when run :

export switch_variable=''  && export position_variable='position' 
pipenv run variable_args

print : ['./list_args.py', '', 'position']

if run ./list_args.py ${switch_variable} ${position_variable} directly in sh, get result:

['./list_args.py',  'position']

Expected result

pipenv run shold generate the same result as run in shell.

Actual result

['./list_args.py', '', 'position']

suggest

check string empty in

pipenv/pipenv/core.py

Lines 2462 to 2464 in 3e63f07

	os.execl(
	command_path, command_path, *[os.path.expandvars(arg) for arg in script.args]
	)

 os.execl(
        command_path, command_path, *[os.path.expandvars(arg) for arg in script.args if  os.path.expandvars(arg) ]
    )

Steps to replicate

Provide the steps to replicate (which usually at least includes the commands and the Pipfile).

---

$ pipenv --support

Pipenv version: '2018.11.26'

Pipenv location: '/Library/Frameworks/Python.framework/Versions/3.6/lib/python3.6/site-packages/pipenv'

Python location: '/Library/Frameworks/Python.framework/Versions/3.6/bin/python3.6'

Python installations found:

• 3.7.3: /usr/bin/python3
• 3.6.6: /Library/Frameworks/Python.framework/Versions/3.6/bin/python3
• 3.6.6: /Library/Frameworks/Python.framework/Versions/3.6/bin/python3.6m
• 2.7.16: /usr/bin/python
• 2.7.16: /usr/bin/pythonw

PEP 508 Information:

{'implementation_name': 'cpython',
 'implementation_version': '3.6.6',
 'os_name': 'posix',
 'platform_machine': 'x86_64',
 'platform_python_implementation': 'CPython',
 'platform_release': '19.0.0',
 'platform_system': 'Darwin',
 'platform_version': 'Darwin Kernel Version 19.0.0: Thu Oct 17 16:17:15 PDT '
                     '2019; root:xnu-6153.41.3~29/RELEASE_X86_64',
 'python_full_version': '3.6.6',
 'python_version': '3.6',
 'sys_platform': 'darwin'}

System environment variables:

• TERM_SESSION_ID
• SSH_AUTH_SOCK
• LC_TERMINAL_VERSION
• COLORFGBG
• ITERM_PROFILE
• SQLITE_EXEMPT_PATH_FROM_VNODE_GUARDS
• XPC_FLAGS
• LANG
• PWD
• SHELL
• TERM_PROGRAM_VERSION
• TERM_PROGRAM
• PATH
• DISPLAY
• LC_TERMINAL
• COLORTERM
• TERM
• HOME
• TMPDIR
• USER
• XPC_SERVICE_NAME
• LOGNAME
• ITERM_SESSION_ID
• __CF_USER_TEXT_ENCODING
• SHLVL
• OLDPWD
• ZSH
• PAGER
• LESS
• LSCOLORS
• LC_CTYPE
• BITBUCKET_USER
• BITBUCKET_PWD
• LC_ALL
• GOPATH
• http_proxy
• https_proxy
• switch_variable
• position_variable
• _
• PIP_DISABLE_PIP_VERSION_CHECK
• PYTHONDONTWRITEBYTECODE
• PIP_SHIMS_BASE_MODULE
• PIP_PYTHON_PATH
• PYTHONFINDER_IGNORE_UNSUPPORTED

Pipenv–specific environment variables:

Debug–specific environment variables:

• PATH: /Users/key00000/Library/Python/3.6/bin:/Users/key00000/bin:/Library/Frameworks/Python.framework/Versions/3.6/bin:/Applications/VLC.app/Contents/MacOS/:/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:/Library/TeX/texbin:/usr/local/share/dotnet:/opt/X11/bin:~/.dotnet/tools:/Applications/Wireshark.app/Contents/MacOS:/Users/key00000/go/bin
• SHELL: /bin/zsh
• LANG: zh_CN.utf-8
• PWD: /tmp/pipenvvariable

---

Contents of Pipfile ('/private/tmp/pipenvvariable/Pipfile'):

[[source]]
name = "pypi"
url = "https://pypi.org/simple"
verify_ssl = true

[dev-packages]

[packages]

[requires]
python_version = "3.6"
[scripts]
variable_args="./list_args.py ${switch_variable} ${position_variable}"

Contents of Pipfile.lock ('/private/tmp/pipenvvariable/Pipfile.lock'):

{
    "_meta": {
        "hash": {
            "sha256": "415dfdcb118dd9bdfef17671cb7dcd78dbd69b6ae7d4f39e8b44e71d60ca72e7"
        },
        "pipfile-spec": 6,
        "requires": {
            "python_version": "3.6"
        },
        "sources": [
            {
                "name": "pypi",
                "url": "https://pypi.org/simple",
                "verify_ssl": true
            }
        ]
    },
    "default": {},
    "develop": {}
}