server: fix crash if no min version specified

If a CMakeLists.txt file doesn't contain cmake_minimum_required then the
server was crashing. The root cause was the json object model does not
support null and was crashing. Add the null check and use an empty
string in this case.

Source/cmServerProtocol.cxx

@@ -1034,8 +1034,8 @@ static Json::Value DumpProjectList(const cmake* cm, std::string const& config)
 
     // Project structure information:
     const cmMakefile* mf = lg->GetMakefile();
-    pObj[kMINIMUM_CMAKE_VERSION] =
-      mf->GetDefinition("CMAKE_MINIMUM_REQUIRED_VERSION");
+    auto minVersion = mf->GetDefinition("CMAKE_MINIMUM_REQUIRED_VERSION");
+    pObj[kMINIMUM_CMAKE_VERSION] = minVersion ? minVersion : "";
     pObj[kSOURCE_DIRECTORY_KEY] = mf->GetCurrentSourceDirectory();
     pObj[kBUILD_DIRECTORY_KEY] = mf->GetCurrentBinaryDirectory();
     pObj[kTARGETS_KEY] = DumpTargetsList(projectIt.second, config);