Fix when omitempties(T) = true is defined in StructTypes.foreachfield (#50)

I noticed in some serialized JSON from JSON3.jl recently that some
fields were still getting serialized when I had defined
`StructTypes.omitempties(::Type{MyType}) = true`, which is a shorthand
we introduced when you want to omit empty values for any field in a
type. Unfortunately, we failed to update `StructTypes.foreachfield` to
respect this option and double unfortunately, the code still happened to
"work", i.e. not throw an error whether `omitempties` returned a `Bool`
or `Tuple`. The fix is pretty simple: if `omitempties` returns `true`,
then we query the `fieldnames` of the struct.

Author: quinnj

src/StructTypes.jl

@@ -628,7 +628,7 @@ Various "configurations" are respected when applying `f` to each field:
     excl = excludes(T)
     nms = names(T)
     kwargs = keywordargs(T)
-    emp = omitempties(T)
+    emp = omitempties(T) === true ? fieldnames(T) : omitempties(T)
     Base.@nexprs 32 i -> begin
         k_i = fieldname(T, i)
         if !symbolin(excl, k_i) && isdefined(x, i)
@@ -673,7 +673,6 @@ Nothing is returned and results from `f` are ignored. Similar to `Base.foreach`
 Various "configurations" are respected when applying `f` to each field:
   * If keyword arguments have been defined for a field via `StructTypes.keywordargs`, they will be passed like `f(i, name, FT, v; kw...)`
   * If `StructTypes.names` has been defined, `name` will be the serialization name instead of the defined julia field name
-  * If a field is undefined or empty and `StructTypes.omitempties` is defined, `f` won't be applied to that field
   * If a field has been excluded via `StructTypes.excludes`, it will be skipped
 """
 @inline function foreachfield(f, ::Type{T}) where {T}
@@ -682,7 +681,6 @@ Various "configurations" are respected when applying `f` to each field:
     excl = excludes(T)
     nms = names(T)
     kwargs = keywordargs(T)
-    emp = omitempties(T)
     Base.@nexprs 32 i -> begin
         k_i = fieldname(T, i)
         if !symbolin(excl, k_i)

test/runtests.jl

@@ -13,6 +13,10 @@ end
 struct EmptyStruct
 end
 
+struct OmitEmp
+    x::Union{Nothing, Int}
+end
+
 @testset "StructTypes" begin
 
 @test StructTypes.StructType(Union{Int, Missing}) == StructTypes.Struct()
@@ -127,6 +131,18 @@ v = v"1.2.3"
 x = Dict("hey" => "ho")
 @test StructTypes.construct(Dict{String, String}, x) === x
 
+# omitempties(T) = true respected
+StructTypes.StructType(::Type{OmitEmp}) = StructTypes.Struct()
+StructTypes.omitempties(::Type{OmitEmp}) = true
+StructTypes.foreachfield(OmitEmp(1)) do i, nm, T, val
+    @test val == 1
+end
+counter = 0
+StructTypes.foreachfield(OmitEmp(nothing)) do i, nm, T, val
+    counter += 1
+end
+@test counter == 0
+
 end
 
 struct B