Update include/datadog/environment.h

Co-authored-by: Zach Montoya <zach.montoya@datadoghq.com>

Author: khanayan123

include/datadog/environment.h

@@ -82,7 +82,7 @@ namespace environment {
   MACRO(DD_TRACE_RESOURCE_RENAMING_ENABLED, BOOLEAN, false)                    \
   MACRO(DD_TRACE_RESOURCE_RENAMING_ALWAYS_SIMPLIFIED_ENDPOINT, BOOLEAN, false) \
   MACRO(DD_EXTERNAL_ENV, STRING, "")                                           \
-  MACRO(_DD_ROOT_CPP_SESSION_ID, STRING, nullptr)
+  MACRO(_DD_ROOT_CPP_SESSION_ID, STRING, "")
 
 #define ENV_DEFAULT_RESOLVED_IN_CODE(X) X
 #define WITH_COMMA(ARG, TYPE, DEFAULT_VALUE) ARG,