Fix SQLiteAppender in release builds (#179)

Release builds that contain no debug info for stack traces
result in nulls for caller data, causing SQLiteAppender to
attempt INSERT statements with illegal null values.

This patch null-checks those INSERT-bindings to avoid the
exception and allow existing debug-enabled apps to log caller
data to the database.

logback-android/src/main/java/ch/qos/logback/classic/android/SQLiteAppender.java

@@ -481,14 +481,20 @@ private void bindCallerData(SQLiteStatement stmt, StackTraceElement[] callerData
     if (callerDataArray != null && callerDataArray.length > 0) {
       StackTraceElement callerData = callerDataArray[0];
       if (callerData != null) {
-        stmt.bindString(CALLER_FILENAME_INDEX, callerData.getFileName());
-        stmt.bindString(CALLER_CLASS_INDEX, callerData.getClassName());
-        stmt.bindString(CALLER_METHOD_INDEX, callerData.getMethodName());
-        stmt.bindString(CALLER_LINE_INDEX, Integer.toString(callerData.getLineNumber()));
+        bindString(stmt, CALLER_FILENAME_INDEX, callerData.getFileName());
+        bindString(stmt, CALLER_CLASS_INDEX, callerData.getClassName());
+        bindString(stmt, CALLER_METHOD_INDEX, callerData.getMethodName());
+        bindString(stmt, CALLER_LINE_INDEX, Integer.toString(callerData.getLineNumber()));
       }
     }
   }
 
+  private void bindString(SQLiteStatement stmt, int columnIndex, String value) {
+    if (value != null) {
+      stmt.bindString(columnIndex, value);
+    }
+  }
+
   /**
    * Inserts an exception into the logging_exceptions table
    *