removing all proficiencies throws TypeError

[phivk]

When I remove all proficiencies (as suggested in person.yaml), like such

proficiencies:
  # - list item

the build process throws a throws a TypeError, which prevents the resume being built:

{ TypeError: metalsmith-resume/templates/default.pug:32
    30|               +experience(collections.education[i])
    31| 
  > 32|         if person.proficiencies.length > 0
    33|           +section
    34|             +left-column
    35|               +section-title('Proficiencies')

Cannot read property 'length' of null
    at eval (eval at wrap (metalsmith-resume/node_modules/pug-runtime/wrap.js:6:10), <anonymous>:526:25)

I'm on OSX 10.12.6 using latest node (5.5.1)

[phivk]

I found a simple fix in default.pug and submitted this PR:
#17