[INTERNAL][FIX] sap.ui.core.Icon: Don't assert undefined/null "src"
[INTERNAL][FIX] sap.ui.core.Icon: Don't assert undefined/null "src"

undefined/null is passed for setting the default, so it shouldn't cause
an assertion for an invalid Icon URI.

Change-Id: I2f566c73afa828f74adeadf4f0ebff9273799988

SAP/openui5,SAP/openui5,SAP/openui5,SAP/openui5