Bug in handling of maybe_placeholders

[erezsh]

Describe the bug

When missing alternatives within [], Lark will produce None for all the alternatives, while only generating one match if any of them match.

This breaks the principle behind it, the argument count must remain the same whether or not there is a match.

To Reproduce

from lark import Lark

p = Lark(r"""
start: b [c | d]

b: "b"
c: "c"
d: "d"

""")

print(p.parse('b').pretty())
print(p.parse('bc').pretty())

# Output
# ------

# start
#   b
#   None
#   None

# start
#   b
#   c

@MegaIng @ornariece Not sure how we missed it, but we need to fix it.

Related PRs: #922