Handle case where SQL is not saved in map properly.

Otherwise this could result in an NPE.

dd-java-agent/src/main/java/com/datadoghq/agent/instrumentation/jdbc/PreparedStatementInstrumentation.java

@@ -23,6 +23,7 @@ import net.bytebuddy.asm.Advice;
 
 @AutoService(Instrumenter.class)
 public final class PreparedStatementInstrumentation implements Instrumenter {
+  private static final String UNKNOWN_QUERY = "Unknown Query";
 
   @Override
   public AgentBuilder instrument(final AgentBuilder agentBuilder) {
@@ -61,7 +62,7 @@ public final class PreparedStatementInstrumentation implements Instrumenter {
       Tags.COMPONENT.set(span, "java-jdbc-prepared_statement");
 
       span.setTag(DDTags.SERVICE_NAME, dbInfo.getType());
-      span.setTag(DDTags.RESOURCE_NAME, sql);
+      span.setTag(DDTags.RESOURCE_NAME, sql == null ? UNKNOWN_QUERY : sql);
       span.setTag(DDTags.SPAN_TYPE, "sql");
       span.setTag("span.origin.type", statement.getClass().getName());
       span.setTag("db.jdbc.url", dbInfo.getUrl());

dd-trace/src/main/java/com/datadoghq/trace/DDSpanContext.java

@@ -176,9 +176,13 @@ public class DDSpanContext implements io.opentracing.SpanContext {
    * Add a tag to the span. Tags are not propagated to the children
    *
    * @param tag the tag-name
-   * @param value the value of the value
+   * @param value the value of the tag. tags with null values are ignored.
    */
   public synchronized void setTag(final String tag, final Object value) {
+    if (value == null) {
+      return;
+    }
+
     if (tag.equals(DDTags.SERVICE_NAME)) {
       setServiceName(value.toString());
       return;
@@ -210,14 +214,10 @@ public class DDSpanContext implements io.opentracing.SpanContext {
       }
     }
     // Error management
-    if (Tags.ERROR.getKey().equals(tag) && Boolean.TRUE.equals(value)) {
+    if (Tags.ERROR.getKey().equals(tag)
+        && Boolean.TRUE.equals(value instanceof String ? Boolean.valueOf((String) value) : value)) {
       this.errorFlag = true;
     }
-
-    // Remove null values
-    if (value == null) {
-      this.tags.remove(tag);
-    }
   }
 
   public synchronized Map<String, Object> getTags() {

dd-trace/src/test/groovy/com/datadoghq/trace/DDSpanContextTest.groovy

@@ -0,0 +1,7 @@
+package com.datadoghq.trace
+
+import spock.lang.Specification
+
+class DDSpanContextTest extends Specification {
+
+}