[#49] Fixed NPE when credentials not set on Linux

[g3force]

Correctly check for missing credentials value

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[coveralls]

Coverage remained the same at 89.583% when pulling e9ae448 on g3force:#49 into 0263d95 on seu-as-code:master.

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Thank you, the title and description now looks good!

[lreimer]

Published 2.6.0 to Bintray and Gradle Plugin Portal: https://plugins.gradle.org/plugin/de.qaware.seu.as.code.credentials
Fixes #49.