Backup bug fix

[relaxdiego]

Description

This replaces PR #781.

Currently if a backup doesn't exists on a target, we return None, since we're using this value in getting a backup target we end up throwing an error
Also after linting, it looks like paginated responses were erroring out if paginated responses were empty.

Status

ready for review

Checklist (tick everything that applies)

• Code linting (required, can be done after the PR checks)
• Documentation
• Tests
• ICLA (required for bigger changes)

[relaxdiego]

The error from https://travis-ci.org/apache/libcloud/jobs/160522709 seems to be a flaky installation error whereas the error from https://travis-ci.org/apache/libcloud/jobs/160522727:

pylint runtests: commands[0] | pylint -E --rcfile=./.pylintrc libcloud/common/
************* Module libcloud.common.dimensiondata
E:489,18: Instance of 'unicode' has no 'get' member (no-member)
E:490,20: Instance of 'unicode' has no 'get' member (no-member)
E:491,39: Instance of 'unicode' has no 'get' member (no-member)

Appears to be a pylint error. The variable being referred to is an Element, not a unicode, object:

-> yield paged_resp
(Pdb) paged_resp
<Element '{urn:didata.com:api:cloud:types}servers' at 0x1100a7a90>

Compare the last commit's build with the build I ran against my fork and notice that only the pylint error shows up: https://travis-ci.org/relaxdiego/libcloud/builds/160750260

[tonybaloney]

fixed build in downstream PR, waiting for @relaxdiego to merge into this PR.