Fix crash when loading boards

[silvanocerza]

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• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• The PR follows
  our contributing guidelines
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• UPGRADING.md has been updated with a migration guide (for breaking changes)

• What kind of change does this PR introduce?

Fixes a crash when loading boards for certain platforms.

• What is the current behavior?

Loading a platform with a board that declares a custom board option like so:

duemilanove.menu.cpu.atmega328=ATmega328P
duemilanove.menu.cpu.atmega168=ATmega168

but doesn't declare any custom board option name like menu.MENU_ID=Text crashes the CLI.

• What is the new behavior?

Loading a platform with a board that declares a custom board option doesn't crash the CLI even if no option name is declared.

• Does this PR introduce a breaking change, and is
  titled accordingly?

No.

• Other information:

This bug was already fixed by #1326 but reintroduced with #1330.

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