Support empty input paths on PBXShellScriptBuildPhase

[lacasseio]

As a user building Xcode targets using Gradle, I want Gradle to ignore empty inputPaths in PBXShellScriptBuildPhase so that I can build Xcode targets.

It doesn't seem like empty inputPaths should be supported, but the Wikipedia application has some which break our build adapter. Ignoring the paths is probably the best solution.

Acceptance Criteria

• [ ] Empty inputPaths does not participate in the up-to-date check