add new number-or-zero() XPath function

[tiritea]

Returns 0 instead of NaN if argument is not a valid numeric expression, eg number-or-zero(${foo})

Specifically intended to replace the more long-winded coalesce(${foo},0) or if(${foo}!='',${foo},0)

Closes #

What has been done to verify that this works as intended?

Why is this the best possible solution? Were any other approaches considered?

this solution is intended to make more 'complicated' solutions less necessary or prone to user mistakes.

Are there any risks to merging this code? If so, what are they?

Shouldnt be - this new XPath function wont ever get used unless explicitly added to XForm expression by form writer. So will not affect any existing forms.

[tiritea]

First ODK PR! Please be gentle... ;-)

D'oh. will fix...

[tiritea]

stupid mistake

[codecov-io]

Codecov Report

Merging #388 into master will decrease coverage by 0.03%.
The diff coverage is 0%.

@@             Coverage Diff              @@
##             master     #388      +/-   ##
============================================
- Coverage     48.31%   48.27%   -0.04%     
+ Complexity     2869     2868       -1     
============================================
  Files           239      239              
  Lines         13506    13512       +6     
  Branches       2621     2623       +2     
============================================
- Hits           6525     6523       -2     
- Misses         6146     6152       +6     
- Partials        835      837       +2

Impacted Files	Coverage Δ	Complexity Δ
src/org/javarosa/xpath/expr/XPathFuncExpr.java	53.99% <0%> (-0.53%)	158 <0> (ø)
...c/org/javarosa/core/services/PrototypeManager.java	79.16% <0%> (-8.34%)	8% <0%> (-1%)

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[tiritea]

FYI would imply corresponding function added to Enketo XPath (@MartijnR)

[tiritea]

Cancelled per discussion on ODK Forum; https://forum.opendatakit.org/t/new-number-or-zero-xpath-function/15975