title: Pandas .loc doesn't find null values based on .isnull() boolean array
body: <p>I'm trying to get info about the null-values in my DF column <code>LotFrontage</code>. As you can see, there are some of them, confirmed in 2 ways:</p>
  <pre><code>hp_lot = houseprices[['LotFrontage', 'LotArea']]
  hp_lot.describe()
  hp_lot.info()
  </code></pre>
  <p>output:</p>
  <pre><code>&lt;class 'pandas.core.frame.DataFrame'&gt;
  Int64Index: 2919 entries, 1 to 2919
  Data columns (total 2 columns):
   #   Column       Non-Null Count  Dtype  
  ---  ------       --------------  -----  
   0   LotFrontage  2433 non-null   float64
   1   LotArea      2919 non-null   int64  
  dtypes: float64(1), int64(1)
  memory usage: 68.4 KB
  </code></pre>
  <pre><code>houseprices_num['LotFrontage'].isnull().describe()
  </code></pre>
  <p>output:</p>
  <pre><code>count      2919
  unique        2
  top       False
  freq       2433
  </code></pre>
  <p>But when I'm trying to locate them, I'm just getting this:</p>
  <pre><code>lf_null = houseprices_num.loc[houseprices_num['LotFrontage'].isnull(), ['LotFrontage']]
  lf_null.describe()
  </code></pre>
  <p>output:</p>
  <pre><code>count    0.0
  mean     NaN
  std      NaN
  min      NaN
  25%      NaN
  50%      NaN
  75%      NaN
  max      NaN
  Name: LotFrontage, dtype: float64
  </code></pre>
  <p>My question is where the hell are my null-values? And if I messed up something in the syntax, why am I not getting an error message of some kind?</p>
  <p>The variables:</p>
  <pre><code>traindf = pd.read_csv('.\\train.csv', sep=',', header=1, index_col='Id')
  testdf = pd.read_csv('.\\test.csv', sep=',', header=0, index_col='Id')
  houseprices = pd.concat([traindf, testdf], axis=0)
  houseprices_num = houseprices[['LotFrontage', 'LotArea', 'MasVnrArea', 'BsmtFinSF1', 'BsmtFinSF2', 'BsmtUnfSF',
                                'TotalBsmtSF', '1stFlrSF', '2ndFlrSF', 'LowQualFinSF', 'GrLivArea', 'GarageArea', 'WoodDeckSF', 'OpenPorchSF', 'EnclosedPorch',
                                '3SsnPorch', 'ScreenPorch', 'PoolArea', 'MiscVal', 'SalePrice']]
  </code></pre>