PTFM-12309 Adding call to JSON::get<std::string>()
PTFM-12309 Adding call to JSON::get<std::string>()

This fixes an LLVM/Clang build error in the upload agent.

dnanexus/dx-toolkit,jhuttner/dx-toolkit,johnwallace123/dx-toolkit,olegnev/dx-toolkit,andyshinn/dx-toolkit,dnanexus/dx-toolkit,jhuttner/dx-toolkit,jhuttner/dx-toolkit,dnanexus/dx-toolkit,olegnev/dx-toolkit,dnanexus/dx-toolkit,jhuttner/dx-toolkit,johnwallace123/dx-toolkit,jhuttner/dx-toolkit,andyshinn/dx-toolkit,olegnev/dx-toolkit,johnwallace123/dx-toolkit,olegnev/dx-toolkit,olegnev/dx-toolkit,dnanexus/dx-toolkit,dnanexus/dx-toolkit,jhuttner/dx-toolkit,andyshinn/dx-toolkit,johnwallace123/dx-toolkit,olegnev/dx-toolkit,andyshinn/dx-toolkit,andyshinn/dx-toolkit,andyshinn/dx-toolkit,johnwallace123/dx-toolkit,dnanexus/dx-toolkit,olegnev/dx-toolkit,johnwallace123/dx-toolkit,andyshinn/dx-toolkit,olegnev/dx-toolkit,dnanexus/dx-toolkit,johnwallace123/dx-toolkit,jhuttner/dx-toolkit,johnwallace123/dx-toolkit,andyshinn/dx-toolkit,jhuttner/dx-toolkit