chop! no longer produces empty coefficients

[dstahlke]

Test case:
using ApproxFun
x,y = Fun(identity, Chebyshev() * Chebyshev())
ApproxFun.norm(0*x)
ApproxFun.norm(zero(x))

I didn't create a unit test because I couldn't find a way to reproduce with ApproxFunBase only.

[dstahlke]

Without this patch, norm(zero(x)) gives the following error in ApproxFunBase/src/Multivariate/ProductFun.jl:177:

ERROR: LoadError: BoundsError: attempt to access 0×0 Matrix{Float64} at index [1:1, 1:1]

Fixing ProductFun to handle empty coefficients just leads to a similar error later on. So it seems this package generally assumes that the coefficients vector is not empty. Thus I modified chop! to never return an empty coefficients vector.

[jishnub]

Thanks! ideally the empty ProductFun and LowRankFun should be made to work, but that requires fixing a lot of code, so I think this should be fine

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 26.38%. Comparing base (2f401fa) to head (20867a2).
Report is 151 commits behind head on master.

Files with missing lines	Patch %	Lines
src/Fun.jl	0.00%	1 Missing ⚠️

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HEAD has 12 uploads less than BASE

Flag	BASE (2f401fa)	HEAD (20867a2)
	16	4

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #479       +/-   ##
===========================================
- Coverage   71.22%   26.38%   -44.84%     
===========================================
  Files          80       81        +1     
  Lines        8331     8443      +112     
===========================================
- Hits         5934     2228     -3706     
- Misses       2397     6215     +3818     

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