MINOR: [C++] Ensure setting the default CMAKE_BUILD_TYPE (apache#43794)

### Rationale for this change

The current logic for detecting whether the `CMAKE_BUILD_TYPE` is set is incorrect. That variable is never fully undefined; by default, in cases where it is unset is actually set to the empty string. Therefore, the condition that must be checked is not whether the variable is defined, but whether it tests to a truthy value (i.e. is a non-empty string).

I consider this a minor change so I have not opened an associated issue.

### What changes are included in this PR?

This PR changes `if(NOT DEFINED CMAKE_BUILD_TYPE)` to `if(NOT CMAKE_BUILD_TYPE)`.

### Are these changes tested?

Since this fixes a particular CMake build scenario I am not sure if a test is merited, or where one would be added.

### Are there any user-facing changes?

No.

Authored-by: Vyas Ramasubramani <vyasr@nvidia.com>
Signed-off-by: Sutou Kouhei <kou@clear-code.com>

cpp/CMakeLists.txt

@@ -84,7 +84,7 @@ set(ARROW_VERSION "18.0.0-SNAPSHOT")
 string(REGEX MATCH "^[0-9]+\\.[0-9]+\\.[0-9]+" ARROW_BASE_VERSION "${ARROW_VERSION}")
 
 # if no build type is specified, default to release builds
-if(NOT DEFINED CMAKE_BUILD_TYPE)
+if(NOT CMAKE_BUILD_TYPE)
   set(CMAKE_BUILD_TYPE
       Release
       CACHE STRING "Choose the type of build.")

cpp/examples/minimal_build/CMakeLists.txt

@@ -30,7 +30,7 @@ endif()
 # We require a C++17 compliant compiler
 set(CMAKE_CXX_STANDARD_REQUIRED ON)
 
-if(NOT DEFINED CMAKE_BUILD_TYPE)
+if(NOT CMAKE_BUILD_TYPE)
   set(CMAKE_BUILD_TYPE Release)
 endif()