Empty constraint value lost during some cloud-init step

[Impact]
??

[Test Case]
??

[Regression Potential]
Minimal

[Original Bug Report]
There was a problem with the cloud-init userdata part of the boot, where it appears that the running of the scripts removed the quotes. Inside the yaml file, the scripts is an array of strings, and the params look like:
  --constraints ''
Also noteworthy is that the --env-config is similarly quoted, and the command line in the pastebin didn't have the single quotes around that either.

Low importance as we are working around the bug by no longer passing through empty constraints on the command line.

This is from the pastebin:

ubuntu@ta3r4:~$ /var/lib/juju/tools/1.16.0.1-precise-amd64/jujud bootstrap-state --data-dir /var/lib/juju --env-config YWRtaW4tc2VjcmV0OiAiIgphZ2VudC12ZXJzaW9uOiAxLjE2LjAuMQphcGktcG9ydDogMTcwNzAKYXV0aG9yaXplZC1rZXlzOiAnc3NoLXJzYSBBQUFBQjNOemFDMXljMkVBQUFBREFRQUJBQUFCQVFEVmI4cnZNSURYcU8xTVFQdU9HRitZdjhjdUlwUjlUWlVIeVJUQlN0dHYzNEcrWVYzSWZwNkJmTDJmZmhIU0k2YjYrQTFPN2l2T0cxRE1kRGhWNXM4ZWpsY1lKSDYrb0IrUVc1UzZ6Y1pLTUhFeks0aXVybWZYdzlBbTUyS0JaRWtabHc4ZEY4R2p2dDF0aFE0ZjhIa2dJbDRzNytybGo3aGJCVnBpenY5UWl5NS8raXlNS3crQWVoVThSZkZDdjJ0NGREeU10WXJFR2xVV2x4SzZHUkxkekVhWlYvbk9EQnhkeGxBdWZ4T0ZiSkluMURTVE9RY2gxdTFhOStKRFNLZkJDa1J6Y0xWUy9vdm55RCtMbmZSN2o2UlRDWHlSMjcxK1FLRVZ5YjRlOFlMbzlDNTBkeHdxZzNuT3BXalFRUzA5OGdmMitmNVBWUWlBeXRvWnd0OHIKICByb290QGF1dG9wa2d0ZXN0CgonCmNhLWNlcnQ6ICctLS0tLUJFR0lOIENFUlRJRklDQVRFLS0tLS0KCiAgTUlJQ1Z6Q0NBY0tnQXdJQkFnSUJBREFMQmdrcWhraUc5dzBCQVFVd1FqRU5NQXNHQTFVRUNoTUVhblZxZFRFeAoKICBNQzhHQTFVRUF3d29hblZxZFMxblpXNWxjbUYwWldRZ1EwRWdabTl5SUdWdWRtbHliMjV0Wlc1MElDSnRZV0Z6CgogIElqQWVGdzB4TXpFd01USXhNRFUwTURKYUZ3MHlNekV3TVRJeE1EVTVNREphTUVJeERUQUxCZ05WQkFvVEJHcDEKCiAgYW5VeE1UQXZCZ05WQkFNTUtHcDFhblV0WjJWdVpYSmhkR1ZrSUVOQklHWnZjaUJsYm5acGNtOXViV1Z1ZENBaQoKICBiV0ZoY3lJd2daMHdDd1lKS29aSWh2Y05BUUVCQTRHTkFEQ0JpUUtCZ1FDZ3JWdVVlWHJaTktqMDhBZ0pZQ3kzCgogIGFJNmFQYzRLTmhRY2QzbWhIQlpVSDcrRWV1SzhjOXRzb21LZDU0cDJZU0Nid3FMT3RMVUdaTFVFaFZZdUZCLzAKCiAgZHJXK2V4d3VkNWlwWS9XMlVZWGN0VkFWVG5rbzdxT2FzR21Hdmd2d2RVQkpoYTVxZm5DTWVuOFg0VWZ4TTJTdwoKICBMazVVZXdqSGFQMVZ1M1ZhMFp6WkR3SURBUUFCbzJNd1lUQU9CZ05WSFE4QkFmOEVCQU1DQUtRd0R3WURWUjBUCgogIEFRSC9CQVV3QXdFQi96QWRCZ05WSFE0RUZnUVVjdDUwQWw3dDJpY2Y1VVNFNWVPVVV6clJYaDB3SHdZRFZSMGoKCiAgQkJnd0ZvQVVjdDUwQWw3dDJpY2Y1VVNFNWVPVVV6clJYaDB3Q3dZSktvWklodmNOQVFFRkE0R0JBSkJNcnVYcgoKICBzVXJpQVF5N0QvMzBnL0hyelJhRkFjUWVabExQeHFBVVp5aXdzNjkvZGpVK0RyRmloTkU4bkk3RUxnTy8zKzRTCgogIDJBR3ZCUm54V3F2S1VaWEhmQlNDdm9hSHB0dnBqUjNOb3FuNlZDajkzcWNXNGwxM0ZtdjgyU2xXRnZHNjJreVMKCiAgRTJRbVBPdlFoYzVVem85WElnT0JqOGJPRTJNWnBRdkVDU0xwCgogIC0tLS0tRU5EIENFUlRJRklDQVRFLS0tLS0KCicKY2EtcHJpdmF0ZS1rZXk6ICIiCmRlZmF1bHQtc2VyaWVzOiBwcmVjaXNlCmRldmVsb3BtZW50OiBmYWxzZQpmaXJld2FsbC1tb2RlOiBpbnN0YW5jZQppbWFnZS1tZXRhZGF0YS11cmw6ICIiCmxvZ2dpbmctY29uZmlnOiA8cm9vdD49REVCVUcKbWFhcy1zZXJ2ZXI6IGh0dHA6Ly8xOTIuMTY4LjIxLjU6ODAvTUFBUwpuYW1lOiBtYWFzCnNzbC1ob3N0bmFtZS12ZXJpZmljYXRpb246IHRydWUKc3RhdGUtcG9ydDogMzcwMTcKdG9vbHMtdXJsOiAiIgp0eXBlOiBtYWFzCg== --constraints --debug
error: invalid value "--debug" for flag --constraints: malformed constraint "--debug"