source_group: TREE with empty string must be supported

Fixes: #28230

Source/cmSourceGroupCommand.cxx

@@ -133,7 +133,7 @@ bool cmSourceGroupCommand(std::vector<std::string> const& args,
     cm::optional<std::string> Regex;
     cm::optional<ArgumentParser::MaybeEmpty<std::vector<std::string>>>
       FileSets;
-    cm::optional<ArgumentParser::NonEmpty<std::string>> Tree;
+    cm::optional<ArgumentParser::MaybeEmpty<std::string>> Tree;
     cm::optional<ArgumentParser::MaybeEmpty<std::string>> Prefix;
 
     std::map<std::string, std::set<std::string>> FileSetsPerTarget;

Tests/RunCMake/SourceGroup/EmptyTREE.cmake

@@ -0,0 +1,2 @@
+
+source_group(TREE "" FILES)

Tests/RunCMake/SourceGroup/RunCMakeTest.cmake

@@ -2,6 +2,7 @@
 include(RunCMake)
 
 run_cmake(MixedSignatures)
+run_cmake(EmptyTREE)
 run_cmake(WrongRegex)
 run_cmake(IgnoredKeywords)
 run_cmake(MissingFILE_SETS)