Fix @nullable import

Since we changed the package name in 1.10.0 and the previous backport
commit used what was available in 1.9.x and deprecated in 1.10.x

See 82dff93
See gh-3759

Author: jonatan-ivanov

micrometer-core/src/test/java/io/micrometer/core/instrument/step/StepMeterRegistryTest.java

@@ -15,9 +15,9 @@
  */
 package io.micrometer.core.instrument.step;
 
+import io.micrometer.common.lang.Nullable;
 import io.micrometer.core.Issue;
 import io.micrometer.core.instrument.*;
-import io.micrometer.core.lang.Nullable;
 import org.junit.jupiter.api.Test;
 import org.testcontainers.shaded.com.google.common.util.concurrent.AtomicDouble;