Werk #1729: apc_symmetra_test: Cleaned up check, fixed exception when self test date is zero

Component	Checks & agents
Title	apc_symmetra_test: Cleaned up check, fixed exception when self test date is zero
Date	Dec 9, 2014
Level	Trivial Change
Class	Bug Fix
Compatibility	Compatible - no manual interaction needed
Checkmk versions & editions	1.2.6b1 Checkmk Raw (CRE), Checkmk Enterprise (CEE), Checkmk MSP (CME)

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