c2SoftAacEnc: fix bad optional access of frame pts

In scenarios, when only eos is signalled with out any input, optional
variable is accessed when it has no value during pts computations. This
is addressed.

Bug: 396505100
Test: atest MctsMediaV2TestCases:AudioEncoderTest
Test: atest MctsMediaV2TestCases:CodecEncoderTest
Test: atest MctsMediaEncoderTestCases:EncoderTest
Test: atest MctsMediaV2TestCases:CodecEncoderBlockModelTest
Test: atest MctsMediaV2TestCases:CodecEncoderMultiAccessUnitTest
Test: atest MctsMediaV2TestCases:CodecEncoderBlockModelMultiAccessUnitTest
Test: atest VtsHalMediaC2V1_0TargetAudioEncTest

Change-Id: I34416fab79e6cc38751595f05b15d6d8861d5df0