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ARROW-4051: [Gandiva] [GLib] Add support for null literal

- Add `#GGandivaNullLiteralNode`.
- Remove `return_type` property in `#GGandivaFunctionNode` to use `ggandiva_node_get_return_type()`.

Author: Yosuke Shiro <yosuke.shiro615@gmail.com>
Author: Kouhei Sutou <kou@clear-code.com>

Closes #3197 from shiro615/glib-support-null-literal-node and squashes the following commits:

4f0a39f7 <Kouhei Sutou> Fix a typo
e93cd085 <Kouhei Sutou> Simplify
be30df17 <Kouhei Sutou> Add tests for return-type of literal nodes
8b3244e7 <Kouhei Sutou> Use data type from GArrowField as return type
fcc0d8f8 <Kouhei Sutou> Raise error for invalid input
41aada5e <Yosuke Shiro> Fix variable names
415df817 <Yosuke Shiro> Add return-type property in ggandiva_field_node_new_raw()
c337f122 <Yosuke Shiro> Call g_object_unref(data_type) to prevent a memory leak
64cef1d8 <Yosuke Shiro> Use the given return_type to create GGandivaNullLiteralNode
b17f5e25 <Yosuke Shiro> Reuse return-type property instead of defining ggandiva_node_get_return_type()
3e25b0d5 <Yosuke Shiro> Refactor null check
0ac03b4d <Yosuke Shiro> Add missing null check
285f64b8 <Yosuke Shiro> Fix orders of class
49d1044a <Yosuke Shiro> Remove return_type property for using ggandiva_node_get_return_type()
f78881cf <Yosuke Shiro> Add ggandiva_node_get_return_type()
5896f0bb <Yosuke Shiro>  Add GGandivaNullLiteralNode