Fixed open/close index api when using wildcard only

Named wildcards were not always properly replaced with proper values by PathTrie.
Delete index (curl -XDELETE localhost:9200/*) worked anyway as the named wildcard is the last path element (and even if {index} didn't get replaced with '*', the empty string would have mapped to all indices anyway). When the named wildcard wasn't the last path element (e.g. curl -XPOST localhost:29200/*/_close), the variable didn't get replaced with the current '*' value, but with the empty string, which leads to an error as empty index is not allowed by open/close index.

Closes #4564