title: Issue with if statement in tradingview pine script
body: <p>i try to put in a simplified switch-statement in my tradingview pine script:</p>
  
  <pre><code>//@version=3
  study("my_test",shorttitle="bands",overlay=true)
  
  string VOLA_INDEX = ""
  
  if (ticker == "USOIL")
     VOLA_INDEX := "OVX"
  if (ticker == "GOLD")
     VOLA_INDEX := "GVZ"
  if (ticker == "GER30")
     VOLA_INDEX := "DV1X"   
  
  
  src = security(ticker,"D",close[1])
  vola = security(VOLA_INDEX,"D",close[1])
  
  bands1 = src * vola/100 * sqrt(0.00273972602) 
  bands3 = src * vola/100 * sqrt(0.00821917808) 
  
  upper1 = src + bands1
  lower1 = src - bands1
  
  plot( src, title="mean", color=black, style=linebr, linewidth=2, transp=100, trackprice = true,offset=-9999)
  plot( upper1, title="upper", color=blue, style=linebr, linewidth=2, transp=40, trackprice = true,offset=-9999)
  plot( lower1, title="lower", color=blue, style=linebr, linewidth=2, transp=40, trackprice = true,offset=-9999)
  </code></pre>
  
  <p>somehow, this could fails. </p>
  
  <p>Someone an idea whats wrong with the syntax?</p>
  
  <p>Thank you</p>