fix SNS Publish exception when using empty topic

[bentsku]

Motivation

Found out this small issue when launching a benchmark without the initial setup. When calling SNS.Publish(Message="test", TopicArn=""), boto accepts it but it fails on LocalStack side.

Changes

• add a test for the issue
• add a small check to use an empty string if the value is falsy

[coveralls]

coverage: 85.675% (-0.008%) from 85.683%
when pulling fa02bfc on fix-sns-no-topic
into 0b9c6b1 on master.

[github-actions]

LocalStack Community integration with Pro

    2 files  ±0      2 suites  ±0   1h 29m 49s ⏱️ - 1m 1s
2 694 tests ±0  2 438 ✅ ±0  256 💤 ±0  0 ❌ ±0 
2 696 runs  ±0  2 438 ✅ ±0  258 💤 ±0  0 ❌ ±0 

Results for commit fa02bfc. ± Comparison against base commit 0b9c6b1.

[thrau]

LGTM!