Fix regex for optional float in parse_mu

[sidey79]

paese_MU:
fixed regex if float was not found as pattern. fixex 596

• Please check if the PR fulfills these requirements

• Tests for the changes have been added / modified (needed for for bug fixes / features)
• commandref has been added / updated (needed for bug fixes / features)
• CHANGED has been updated (needed for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
  Bugfix of regex in parse_MU

• What is the current behavior? (You can also link to an open issue here)
  If float is not found, the part of the regex instead is added.
  This causes that the regex ends with | whcih allows the capture group that no chat dies count for min_length

• What is the new behavior (if this is a feature change)?

If a key (zero / float) is not returned, the regex will not be extendet.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

Nothing known.

• Other information:
  fixes length 0 are dispatch & length_min in ID definition #596