fix: Can't render pages on older versions of Nuxt if options.watch is missing

[arikw]

If nuxt.config.js doesn't contain a watch property, Nuxt v1.4.2 fails to render a page with a TypeError: Cannot read property 'push' of undefined error.

This PR adds an empty watch array to the options as a fallback to fix the issue.

Tested on Nuxt v1.4.2 and v2.14.6

[codecov]

Codecov Report

Merging #52 (d896be4) into master (3e53050) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #52   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           22        23    +1     
  Branches         2         3    +1     
=========================================
+ Hits            22        23    +1     

Impacted Files	Coverage Δ
lib/module.js	100.00% <100.00%> (ø)

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[ricardogobbosouza]

@arikw thanks