[operator] Setting multiple feature gates causes operator panic

[swiatekm]

When we set multiple feature gates using featureGatesMap, we get a malformed command line argument for the operator, leading to a panic.

Using the following values file:

manager:
  featureGatesMap:
    operator.targetallocator.mtls: false
    operator.collector.targetallocatorcr: false
    operator.sidecarcontainers.native: true
    operator.observability.prometheus: false
    operator.golang.flags: true
    operator.collector.default.config: false

results in the operator getting the following command line argument:

--feature-gates=,,operator.golang.flags,,operator.sidecarcontainers.native,

This is an invalid value, and causes the operator to crash. At a glance, this is caused by the fact that we add a comma to the feature gate string even if the value in the map is false.

Originally reported in open-telemetry/opentelemetry-operator#3559.