don't upload cache if password is not set

[drodin]

• I've checked this Git style guide. [Yes]
• I've checked this CMake style guide. [Yes]
• My change will work with CMake 3.2 (minimum requirement for Hunter). [Yes]
• I will try to keep this pull request as small as possible and will try not to mix unrelated features. [Yes]

Helps to mitigate problem with manual workflow run in forked repo (as it's done on the master branch).
Without this change manual package test result in error:
[hunter ** FATAL ERROR **] Upload: PASSWORD is missing

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[rbsheth]

Released: https://github.com/cpp-pm/hunter/releases/tag/v0.23.313