Stable Config parsing: Treat empty environment variable or process argument values as empty string (#8759)

Author: mtoffl01

internal-api/src/main/java/datadog/trace/bootstrap/config/provider/StableConfigParser.java

@@ -21,7 +21,7 @@ public class StableConfigParser {
 
   private static final String ENVIRONMENT_VARIABLES_PREFIX = "environment_variables['";
   private static final String PROCESS_ARGUMENTS_PREFIX = "process_arguments['";
-  private static final String UNDEFINED_VALUE = "UNDEFINED";
+  private static final String UNDEFINED_VALUE = "";
 
   /**
    * Parses a configuration file and returns a stable configuration object.

internal-api/src/test/groovy/datadog/trace/bootstrap/config/provider/StableConfigParserTest.groovy

@@ -190,13 +190,13 @@ apm_configuration_rules:
     where:
     templateVar                                                                                                 | envKey   | envVal  | expect
     "{{environment_variables['DD_KEY']}}"                                                                       | "DD_KEY" | "value" | "value"
-    "{{environment_variables['DD_KEY']}}"                                                                       | null     | null    | "UNDEFINED"
-    "{{}}"                                                                                                      | null     | null    | "UNDEFINED"
+    "{{environment_variables['DD_KEY']}}"                                                                       | null     | null    | ""
+    "{{}}"                                                                                                      | null     | null    | ""
     "{}"                                                                                                        | null     | null    | "{}"
     "{{environment_variables['dd_key']}}"                                                                       | "DD_KEY" | "value" | "value"
-    "{{environment_variables['DD_KEY}}"                                                                         | "DD_KEY" | "value" | "UNDEFINED"
+    "{{environment_variables['DD_KEY}}"                                                                         | "DD_KEY" | "value" | ""
     "header-{{environment_variables['DD_KEY']}}-footer"                                                         | "DD_KEY" | "value" | "header-value-footer"
-    "{{environment_variables['HEADER']}}{{environment_variables['DD_KEY']}}{{environment_variables['FOOTER']}}" | "DD_KEY" | "value" | "UNDEFINEDvalueUNDEFINED"
+    "{{environment_variables['HEADER']}}{{environment_variables['DD_KEY']}}{{environment_variables['FOOTER']}}" | "DD_KEY" | "value" | "value"
   }
 
   def "test processTemplate error cases"() {