title: str.contains based on another column is returning values for empty strings
body: <p>I need col1 in the dataframe to be filled based on strings in col2, such as:</p>
  
  <pre><code>Out[]: 
     Col2      Col1 
  0  SAP       SAP     
  1  Oracle    Oracle     
  2            N/A     
  3  SAP       SAP     
  4  SF        Salesforce     
  </code></pre>
  
  <p>The code works, but col1 has values that don't belong because col2 is blank. (See row 3)</p>
  
  <pre><code>Out[]: 
     Col2      Col1 
  0  SAP       SAP     
  1  Oracle    Oracle     
  2            Oracle    
  3  SAP       SAP     
  4  SF        Salesforce     
  </code></pre>
  
  <p>Tried adding a <code>pd.np.where(df[column2]str.contains(""), "Empty")</code></p>
  
  <p>But then all of column1 is filled with "Empty" and it ignores the rest.</p>
  
  <p>Here's my code:</p>
  
  <pre><code>DF['Col1']= pd.np.where(DF['Col2'].str.contains('Oracle'), 'Oracle',
              pd.np.where(DF['Col2'].str.contains('SAP'), 'SAP',        
              pd.np.where(DF['Col2'].str.contains('SF'), 'Salesforce', 'N/A')))
  </code></pre>
  
  <p>Expected Output is:</p>
  
  <pre><code>Out[]: 
     Col2      Col1 
  0  SAP       SAP      
  1  Oracle    Oracle     
  2            N/A    
  3  SAP       SAP      
  4  SF        Salesforce     
  </code></pre>