Prevent some spurious error messages in the debugserver logs.

DNBGetDeploymentInfo was calling GetPlatformString w/o checking that
the load command it was processing actually provided a platform string.
That caused a bunch of worrisome looking error messages in the debugserver
log output.

Differential Revision: https://reviews.llvm.org/D151861

lldb/tools/debugserver/source/DNB.cpp

@@ -1456,9 +1456,13 @@ DNBGetDeploymentInfo(nub_process_t pid, bool is_executable,
     major_version = info.major_version;
     minor_version = info.minor_version;
     patch_version = info.patch_version;
+    // MachProcess::DeploymentInfo has a bool operator to tell whether we have
+    // set the platform.  If that's not true, don't report out the platform:
+    if (!info)
+      return {};
     return procSP->GetPlatformString(info.platform);
   }
-  return nullptr;
+  return {};
 }
 
 // Get the current shared library information for a process. Only return