Fix '&=' operator optimization when the added string is void.

[INTERPRETER]
* BUG: Fix '&=' operator optimization when the added string is void.

comp/src/gb.form/.src/Test/Form1.class

@@ -0,0 +1,33 @@
+' Gambas class file
+
+Public Sub Run() As Boolean
+
+  Return Not Me.ShowModal()
+
+End
+
+Public Sub btnOK_Click()
+
+  Me.Close(True)
+
+End
+
+Public Sub btnCancel_Click()
+
+  Me.Close
+
+End
+
+
+Public Sub Form_Open()
+
+   Me.Center
+   
+End
+
+
+Public Sub DateBox1_Change()
+
+  Message.Question("does it pass, on debug?")
+
+End

comp/src/gb.form/.src/Test/Form1.form

@@ -0,0 +1,30 @@
+# Gambas Form File 3.0
+
+{ Form Form
+  MoveScaled(0,0,64,64)
+  Resizable = False
+  Arrangement = Arrange.Vertical
+  Spacing = True
+  Margin = True
+  { DateBox1 DateBox
+    MoveScaled(17,17,24,4)
+  }
+  { HBox1 HBox
+    MoveScaled(1,59,62,4)
+    Spacing = True
+    { Panel1 Panel
+      MoveScaled(4,0,4,4)
+      Expand = True
+    }
+    { btnOK Button
+      MoveScaled(29,0,16,4)
+      Text = ("OK")
+      Default = True
+    }
+    { btnCancel Button
+      MoveScaled(46,0,16,4)
+      Text = ("Cancel")
+      Cancel = True
+    }
+  }
+}