Handling undefined function in timers

[olegbespalov]

What?

This PR fixes a panic that could be caused by a script like:

export default function() {
	setInterval(undefined, 100);
}

Why?

We shouldn't panic

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 70.88%. Comparing base (bd114fd) to head (ec26402).

❗ Current head ec26402 differs from pull request most recent head 5819f62

Please upload reports for the commit 5819f62 to get more accurate results.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3776      +/-   ##
==========================================
+ Coverage   70.87%   70.88%   +0.01%     
==========================================
  Files         291      291              
  Lines       21227    21229       +2     
==========================================
+ Hits        15044    15049       +5     
+ Misses       5218     5215       -3     
  Partials      965      965              

Flag	Coverage Δ
ubuntu	70.81% <100.00%> (+<0.01%)	⬆️
windows	70.74% <100.00%> (-0.01%)	⬇️

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