Fix uninitialized variable errors / warnings

[lukasmoellerch]

A build of mine was failing when configuring using the following preset:

 {
      "name": "dev",
      "binaryDir": "${sourceDir}/build",
      "generator": "Unix Makefiles",
      "warnings": {
        "dev": true,
        "deprecated": true,
        "uninitialized": true,
        "unusedCli": true,
        "systemVars": false
      },
      "errors": {
        "dev": true,
        "deprecated": true
      },
      "cacheVariables": {
        "CMAKE_BUILD_TYPE": "Release",
      }
    }

this was caused mainly due to uninitialized variables in the conan cmake module, which should be fixed here.

I am using the find_package generator and it uses the following function:

function(conan_message MESSAGE_OUTPUT)
    if(NOT CONAN_CMAKE_SILENT_OUTPUT)
        message(${ARGV${0}})
    endif()
endfunction()

in this case ${0} is undefined (I am also not sure where it would be defined), thus I defined it here - shall I create a PR to the main cmake repo instead?

I am new to cmake - any feedback is greatly appreciated!

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