BUG: DecimalArray and JSONArray that are empty return incorrect results for isna()
#### Code Sample, a copy-pastable example if possible

```python
In [1]: import pandas as pd

In [2]: from pandas.tests.extension.decimal.array import DecimalArray

In [3]: from pandas.tests.extension.json.array import JSONArray

In [4]: DecimalArray([]).isna()
Out[4]: array([], dtype=float64)

In [5]: JSONArray([]).isna()
Out[5]: array([], dtype=float64)

In [6]: import decimal

In [7]: DecimalArray([decimal.Decimal(1.0)]).isna()
Out[7]: array([False])

In [8]: DecimalArray([decimal.Decimal(1.0)]).isna().dtype
Out[8]: dtype('bool')
```
#### Problem description

This relates to a discussion with @TomAugspurger in #21183 when I made (since redacted) changes to `pandas/util/testing.py` in `assert_extension_array_equal` .  It turns out that the implementations of `isna()` in `DecimalArray` and `JSONArray` return an `ndarray` of the wrong dtype if the arrays are empty.

So we need to force the dtype of the result of `isna()` to be `bool` in those implementations

#### Expected Output

```
In [4]: DecimalArray([]).isna()
Out[4]: array([], dtype=bool)

In [5]: JSONArray([]).isna()
Out[5]: array([], dtype=bool)

```

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.23.0
pytest: 3.3.2
pip: 9.0.1
setuptools: 38.4.0
Cython: 0.27.3
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.6
patsy: 0.5.0
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.2
openpyxl: 2.4.10
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.1
pymysql: 0.7.11.None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
