Some creatures lack certain ability scores. These creatures do not have an ability score of 0 -- they lack the ability altogether. The modifier for a nonability is +0. Other effects of nonabilities are detailed below.
Strength: Any creature that can physically manipulate other objects has at least 1 point of Strength. A creature with no Strength score can't exert force, usually because it has no physical body (a spectre, for example) or because it doesn't move (a shrieker). The creature automatically fails Strength checks. If the creature can attack, it applies its Dexterity modifier to its base attack bonus instead of a Strength modifier.
Dexterity: Any creature that can move has at least 1 point of Dexterity. A creature with no Dexterity score can't move (a shrieker, for example). If it can perform actions (such as casting spells), it applies its Intelligence modifier to initiative checks instead of a Dexterity modifier. The creature automatically fails Reflex saves and Dexterity checks.
Constitution: Any living creature has at least 1 point of Constitution. A creature with no Constitution has no body (a spectre, for example) or no metabolism (a golem). It is immune to any effect that requires a Fortitude save unless the effect works on objects or is harmless. For example, a zombie is unaffected by any type of poison but is susceptible to a disintegrate spell. The creature is also immune to ability damage, ability drain, and energy drain, and automatically fails Constitution checks. A creature with no Constitution cannot tire and thus can run indefinitely without tiring (unless the creature's description says it cannot run).
Intelligence: Any creature that can think, learn, or remember has at least 1 point of Intelligence. A creature with no Intelligence score is mindless, an automaton operating on simple instincts or programmed instructions. It has immunity to mind-affecting effects (charms, compulsions, phantasms, patterns, and morale effects) and automatically fails Intelligence checks. Mindless creatures do not gain feats or skills, although they may have bonus feats or racial skill bonuses.
Wisdom: Any creature that can perceive its environment in any fashion has at least 1 point of Wisdom. Anything with no Wisdom score is an object, not a creature. Anything without a Wisdom score also has no Charisma score.
Charisma: Any creature capable of telling the difference between itself and things that are not itself has at least 1 point of Charisma. Anything with no Charisma score is an object, not a creature. Anything without a Charisma score also has no Wisdom score.