Fix broken gcov command

The gcov command can't be generated if none of the gcov options are
given on the command line, since sanitizeVar() will throw an exception
when given 'undefined' as input. This allows sanitizeVar() to accept an
input of 'undefined'.

lib/codecov.js

@@ -556,6 +556,9 @@ var upload = function(args, on_success, on_failure) {
 }
 
 function sanitizeVar(arg) {
+  if (!arg) {
+    return ''
+  }
   return arg.replace(/&/g, '')
 }
 

test/index.test.js

@@ -282,4 +282,10 @@ describe('Codecov', function() {
       'real  run unsafe  command'
     )
   })
+
+  it('gracefully sanitizes undefined', function() {
+    expect(function() {
+      codecov.sanitizeVar(undefined)
+    }).not.toThrow()
+  })
 })