If/If/Else can lead to no viable alt at EOF

[sharwell]

Here is a simplified grammar demonstrating the problem:

grammar T;
s : a | 'x';
a : 'a' s ('b' s)?;

No output for input axbx (correct)
No output for input x (correct)

Output for input ax (unexpected)

line 1:2 no viable alternative at input '<EOF>'

Output for input aaxbx (unexpected)

line 1:5 no viable alternative at input '<EOF>'

[parrt]

EOF cannot follow s. would need s' : s EOF ; for that. updating unit test TestParserExec.testOptional