Fix empty symbol handling and tape growth in ion-bench serialize_all benchmark.

[nirosys]

Issue #, if available: n/a

Description of changes:
This PR fixes two issues found in ion-bench's serialize_all benchmark.

1. While testing the performance regression workflow one of the generated datasets used an empty symbol ('') as a field name. When the the ion benchmark was generating the tape from the generated data, an error in the benchmark's reader considered a >0 length symbol as an indicator that the symbol existed, rather than relying on the returned iERR.
2. When generating the tape for serialization, the tape was not initially cleared. This resulted in the another representation of the dataset to be appended to an existing tape of the dataset, if/when google-benchmark ran another block of iterations. This PR clears the tape for each of the formats implemented.

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