Improve fs::PathStartsWith to handle empty strings (#14877)

`""` does not refer to a proper path, and `fs::PathStartsWith(path, "")` should just return `false`. This is also the case in libraries in other languages where I looked, seems to be common.

The new behavior:
* check early, if `prefix` is empty - return if path is empty or not,
* no special processing for when `path` is empty, the function meets characters in `prefix` and returns false anyway.

src/filesys.cpp

@@ -697,32 +697,43 @@ bool MoveDir(const std::string &source, const std::string &target)
 
 bool PathStartsWith(const std::string &path, const std::string &prefix)
 {
+	if (prefix.empty())
+		return path.empty();
 	size_t pathsize = path.size();
 	size_t pathpos = 0;
 	size_t prefixsize = prefix.size();
 	size_t prefixpos = 0;
 	for(;;){
+		// Test if current characters at path and prefix are delimiter OR EOS
 		bool delim1 = pathpos == pathsize
 			|| IsDirDelimiter(path[pathpos]);
 		bool delim2 = prefixpos == prefixsize
 			|| IsDirDelimiter(prefix[prefixpos]);
 
+		// Return false if it's delimiter/EOS in one path but not in the other
 		if(delim1 != delim2)
 			return false;
 
 		if(delim1){
+			// Skip consequent delimiters in path, in prefix
 			while(pathpos < pathsize &&
 					IsDirDelimiter(path[pathpos]))
 				++pathpos;
 			while(prefixpos < prefixsize &&
 					IsDirDelimiter(prefix[prefixpos]))
 				++prefixpos;
+			// Return true if prefix has ended (at delimiter/EOS)
 			if(prefixpos == prefixsize)
 				return true;
+			// Return false if path has ended (at delimiter/EOS)
+            // while prefix did not.
 			if(pathpos == pathsize)
 				return false;
 		}
 		else{
+			// Skip pairwise-equal characters in path and prefix until
+			// delimiter/EOS in path or prefix.
+			// Return false if differing characters are met.
 			size_t len = 0;
 			do{
 				char pathchar = path[pathpos+len];