title: postgresql empty string has length 1
body: <p>I have a column type text which looks like empty string but the length of it is 1.</p>
  <p>the following sql</p>
  <pre class="lang-sql prettyprint-override"><code>select 
  teilnetz_name, 
  length(teilnetz_name), 
  trim(teilnetz_name), 
  length(trim(teilnetz_name))
  from test_table
  </code></pre>
  <p>results in</p>
  <pre class="lang-sql prettyprint-override"><code>teilnetz_name| length| btrim| length
               |  1    |      | 1
  </code></pre>
  <p>and</p>
  <pre class="lang-sql prettyprint-override"><code>select case when trim(teilnetz_name) is distinct from '' then true else false end 
  from test_table
  
  --return true
  
  select case when teilnetz_name is distinct from null then true else false end
  from test_table
  
  --return true
  
  select case when teilnetz_name is distinct from '' then true else false end
  from test_table
  
  --return true
  
  select case when teilnetz_name is distinct from ' ' then true else false end
  from test_table
  
  --return true
  </code></pre>
  <p>How can I explain this phenomenon ?</p>
  <p>I have on postgreql version 12.3</p>