fix(sequenceDiagram): allow empty message after colon (Fixes mermaid-js#6518)

Author: NourBenz

packages/mermaid/src/diagrams/sequence/parser/sequenceDiagram.jison

@@ -84,7 +84,8 @@ accDescr\s*"{"\s*                                { this.begin("acc_descr_multili
 \-\-[x]                                                         return 'DOTTED_CROSS';
 \-[\)]                                                          return 'SOLID_POINT';
 \-\-[\)]                                                        return 'DOTTED_POINT';
-":"(?:(?:no)?wrap:)?[^#\n;]+                                    return 'TXT';
+":"(?:(?:no)?wrap:)?[^#\n;]*                                    return 'TXT';
+":"                             								return 'TXT';
 "+"                                                             return '+';
 "-"                                                             return '-';
 <<EOF>>                                                         return 'NEWLINE';