Update IntegerFormatter.java for negative squelch value (#2187)

Two issues:
(1)documentation for change.getText() says it can be an empty string but never null. 
(2)Need to allow for typing a minus sign before validating min/max.

src/main/java/io/github/dsheirer/gui/control/IntegerFormatter.java

@@ -85,7 +85,7 @@ public class IntegerFormatter extends TextFormatter<Integer>
         public Change apply(Change change)
         {
             //Only validate if the user added text to the control.  Otherwise, allow it to go through
-            if(change.getText() != null)
+            if(!change.getText().equals(""))
             {
                 String updatedText = change.getControlNewText();
 
@@ -93,6 +93,12 @@ public class IntegerFormatter extends TextFormatter<Integer>
                 {
                     return change;
                 }
+                
+                // don't validate yet if input is only a minus sign
+                if(updatedText.equals("-"))
+                {
+                    return change;
+                }
 
                 if(!updatedText.matches(DECIMAL_REGEX) || !isValid(updatedText))
                 {