Fix #3767: NPE while scouting if AtB tries to generate enemy with SPAs not found in MM

[Sleet01]

Makes this lookup safer if defaultspa.xml contains an SPA that MM doesn't know about.
This appears to happen when AtB tries to generate a bot force during scouting, if the force will have pilots with SPAs.
About 27% of the options in defaultspa.xml will cause an NPE, so this change just forces a re-roll if the current SPA isn't found.

Testing:

• Ran previously-failing scouting missions from both posted issues.
• Ran all three projects' unit tests.

Close #3767
Close #3884

[IllianiCBT]

You might already done that, but if you haven’t it might be worth adding a log warning to say that MM encountered a SPA it didn’t know about and what SPA it was. I can’t read your code, as I’m on the road, so ignore this if you’ve already got that :)

[Sleet01]

You might already done that, but if you haven’t it might be worth adding a log warning to say that MM encountered a SPA it didn’t know about and what SPA it was. I can’t read your code, as I’m on the road, so ignore this if you’ve already got that :)

Great minds think alike!
I added that as a warning in the MekHQ log.

[AaronGullickson]

LGTM