Add a no argument constructor for JavaObjects

[mkitti]

This change makes it easier to construct objects that require no arguments.

julia> using JavaCall
[ Info: Precompiling JavaCall [494afd89-becb-516b-aafa-70d2670c0337]
┌ Warning: JavaCall needs the environment variable `JULIA_COPY_STACKS` to be `1` or `yes`.
│ Calling the JVM may result in undefined behavior.
└ @ JavaCall ~/.julia/dev/JavaCall/src/JavaCall.jl:42

julia> JavaCall.addClassPath.(["jars/*","plugins/*"]); JavaCall.init()

julia> ImageJ2 = @jimport net.imagej.ImageJ
JavaObject{Symbol("net.imagej.ImageJ")}

julia> ImageJ1 = @jimport ij.ImageJ
JavaObject{Symbol("ij.ImageJ")}

julia> ImageJ2()
JavaObject{Symbol("net.imagej.ImageJ")}(Ptr{Nothing} @0x0000560b5e6c5298)

julia> ImageJ1()
JavaObject{Symbol("ij.ImageJ")}(Ptr{Nothing} @0x0000560b5e6c52a8)

Otherwise, we have to use the weird Tuple syntax:

julia> using JavaCall
┌ Warning: JavaCall needs the environment variable `JULIA_COPY_STACKS` to be `1` or `yes`.
│ Calling the JVM may result in undefined behavior.
└ @ JavaCall ~/.julia/dev/JavaCall/src/JavaCall.jl:42

julia> JavaCall.addClassPath.(["jars/*","jars/bio-formats/*","jars/linux64/*","plugins/*"]); JavaCall.init()

julia> ImageJ2 = @jimport net.imagej.ImageJ
JavaObject{Symbol("net.imagej.ImageJ")}

julia> ImageJ1 = @jimport ij.ImageJ
JavaObject{Symbol("ij.ImageJ")}

julia> ImageJ2((),)
JavaObject{Symbol("net.imagej.ImageJ")}(Ptr{Nothing} @0x00005600d46bf0d8)

julia> ImageJ1((),)
JavaObject{Symbol("ij.ImageJ")}(Ptr{Nothing} @0x00005600d46bf0e8)