fix: allow changing counter for prefix-less series (#19180) (#19190)

(cherry picked from commit 2ab543f)

Co-authored-by: Dany Robert <danyrt@wahni.com>

frappe/core/doctype/document_naming_settings/document_naming_settings.py

@@ -165,15 +165,15 @@ def get_options(self, doctype=None):
 	@frappe.whitelist()
 	def get_current(self):
 		"""get series current"""
-		if self.prefix:
+		if self.prefix is not None:
 			self.current_value = NamingSeries(self.prefix).get_current_value()
 		return self.current_value
 
 	@frappe.whitelist()
 	def update_series_start(self):
 		frappe.only_for("System Manager")
 
-		if not self.prefix:
+		if self.prefix is None:
 			frappe.throw(_("Please select prefix first"))
 
 		naming_series = NamingSeries(self.prefix)
@@ -193,7 +193,7 @@ def update_series_start(self):
 	def create_version_log_for_change(self, series, old, new):
 		version = frappe.new_doc("Version")
 		version.ref_doctype = "Series"
-		version.docname = series
+		version.docname = series or ".#"
 		version.data = frappe.as_json({"changed": [["current", old, new]]})
 		version.flags.ignore_links = True  # series is not a "real" doctype
 		version.flags.ignore_permissions = True