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ARROW-15665: [C++] Fix error_is_null in strptime with invalid inputs

As reported by @dragosmg the following:
```
StrptimeOptions options("%m/%d/%Y", TimeUnit::MICRO, /*error_is_null=*/true);
std::string input2 = R"(["5-1-2020", "12/13/1900"])";
std::string output2 = R"([null, "1900-12-13"])";
Strptime(input2, options)
```
Would cause:
```
Expected:
  [
    null,
    1900-12-13 00:00:00.000000
  ]
Actual:
  [
    null,
    1970-01-01 00:00:00.000000
  ]
```
This was due to out_data pointer not updating on failed parsing.

This change fixes the issue and adds a test.

Closes #12732 from rok/ARROW-15665-follow-up

Authored-by: Rok <rok@mihevc.org>
Signed-off-by: David Li <li.davidm96@gmail.com>