Fix TimeFreqSupport.deep_copy() error

Author: PProfizi

src/ansys/dpf/core/time_freq_support.py

@@ -529,7 +529,8 @@ def deep_copy(self, server=None):
         tf_copy : TimeFreqSupport
         """
         tf = TimeFreqSupport(server=server)
-        tf.time_frequencies = self.time_frequencies.deep_copy(server=server)
+        if self.time_frequencies:
+            tf.time_frequencies = self.time_frequencies.deep_copy(server=server)
         if self.complex_frequencies:
             tf.complex_frequencies = self.complex_frequencies.deep_copy(server=server)
         if self.rpms: