treat null and empty string equally for endpoint properties

This improves the usability of the docker image, as it defaults
to empty strings for both properties. With this fix, the docker image
will work when only one of the endpoints are set.

src/main/java/org/gaul/s3proxy/S3Proxy.java

@@ -161,16 +161,18 @@ public final class S3Proxy {
                     S3ProxyConstants.PROPERTY_ENDPOINT);
             String secureEndpoint = properties.getProperty(
                     S3ProxyConstants.PROPERTY_SECURE_ENDPOINT);
-            if (endpoint == null && secureEndpoint == null) {
+            boolean hasEndpoint = !Strings.isNullOrEmpty(endpoint);
+            boolean hasSecureEndpoint = !Strings.isNullOrEmpty(secureEndpoint);
+            if (!hasEndpoint && !hasSecureEndpoint) {
                 throw new IllegalArgumentException(
                         "Properties file must contain: " +
                         S3ProxyConstants.PROPERTY_ENDPOINT + " or " +
                         S3ProxyConstants.PROPERTY_SECURE_ENDPOINT);
             }
-            if (endpoint != null) {
+            if (hasEndpoint) {
                 builder.endpoint(new URI(endpoint));
             }
-            if (secureEndpoint != null) {
+            if (hasSecureEndpoint) {
                 builder.secureEndpoint(new URI(secureEndpoint));
             }