export fails for project with no name

[jmfederico]

When exporting dependencies from a project which is not a library (is not self installable) an error is raised due to it not being available in the calculated dependencies.
https://github.com/frostming/pdm/blob/master/pdm/cli/commands/export.py#L40-L41

A simple solution is to change line https://github.com/frostming/pdm/blob/master/pdm/cli/commands/export.py#L41 to temp.pop(project.meta.name, None).

• I have searched the issue tracker and believe that this is not a duplicate.

Steps to reproduce

• Create a project (pdm init) and remove name and version from pyproject.toml
  • Option to exclude site-packages from python path #100 (comment)
• add any dependency (pdm add flask)
• run pdm export

Actual behavior

❯ pdm export -v
Traceback (most recent call last):
  File "/Users/my_user/.local/bin/pdm", line 8, in <module>
    sys.exit(main())
  File "/Users/my_user/.local/pipx/venvs/pdm/lib/python3.8/site-packages/pdm/core.py", line 68, in __call__
    return self.main(*args, **kwargs)
  File "/Users/my_user/.local/pipx/venvs/pdm/lib/python3.8/site-packages/pdm/core.py", line 100, in main
    raise err.with_traceback(traceback)
  File "/Users/my_user/.local/pipx/venvs/pdm/lib/python3.8/site-packages/pdm/core.py", line 96, in main
    f(options.project, options)
  File "/Users/my_user/.local/pipx/venvs/pdm/lib/python3.8/site-packages/pdm/cli/commands/export.py", line 41, in handle
    temp.pop(project.meta.name)
KeyError: None

Expected behavior

❯ pdm export
click==7.1.2; python_version >= "2.7" and python_version not in "3.0, 3.1, 3.2, 3.3, 3.4" \
    --hash=sha256:dacca89f4bfadd5de3d7489b7c8a566eee0d3676333fbb50030263894c38c0dc \
    --hash=sha256:d2b5255c7c6349bc1bd1e59e08cd12acbbd63ce649f2588755783aa94dfb6b1a```
...

Environment Information

❯ pdm info && pdm info --env
PDM version:        0.8.4
Python Interpreter: /Users/federico/.pyenv/shims/python3.8 (3.8.3)
Project Root:       /Volumes/Development/test-project
{
  "implementation_name": "cpython",
  "implementation_version": "3.8.3",
  "os_name": "posix",
  "platform_machine": "x86_64",
  "platform_release": "19.5.0",
  "platform_system": "Darwin",
  "platform_version": "Darwin Kernel Version 19.5.0: Tue May 26 20:41:44 PDT 2020; root:xnu-6153.121.2~2/RELEASE_X86_64",
  "python_full_version": "3.8.3",
  "platform_python_implementaiton": "CPython",
  "python_version": "3.8",
  "sys_platform": "darwin"
}